[cleanup] Introduce since_origin helper to TimeBase

TimeTicks cannot convert to other timeunits, only TimeDelta can
do that. Chrome's version of TimeBase introduced a `since_origin`
helper that essentially converts a TimeTicks object in to a
TimeDelta with the corresponding duration.

This CL ports that helper to V8 and cleans up a couple of places
where we used `timestamp - 0` to convert a TimeTicks object to
a TimeDelta.

Change-Id: I540ced876ac3e727dfdedf7b838a272cc40d6954
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1993282
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65820}

src/api/api.cc

@@ -10364,18 +10364,17 @@ const CpuProfileNode* CpuProfile::GetSample(int index) const {
 
 int64_t CpuProfile::GetSampleTimestamp(int index) const {
   const i::CpuProfile* profile = reinterpret_cast<const i::CpuProfile*>(this);
-  return (profile->sample(index).timestamp - base::TimeTicks())
-      .InMicroseconds();
+  return profile->sample(index).timestamp.since_origin().InMicroseconds();
 }
 
 int64_t CpuProfile::GetStartTime() const {
   const i::CpuProfile* profile = reinterpret_cast<const i::CpuProfile*>(this);
-  return (profile->start_time() - base::TimeTicks()).InMicroseconds();
+  return profile->start_time().since_origin().InMicroseconds();
 }
 
 int64_t CpuProfile::GetEndTime() const {
   const i::CpuProfile* profile = reinterpret_cast<const i::CpuProfile*>(this);
-  return (profile->end_time() - base::TimeTicks()).InMicroseconds();
+  return profile->end_time().since_origin().InMicroseconds();
 }
 
 int CpuProfile::GetSamplesCount() const {

src/base/platform/time.h

@@ -268,6 +268,16 @@ class TimeBase : public TimeConstants {
   // provided operators.
   int64_t ToInternalValue() const { return us_; }
 
+  // The amount of time since the origin (or "zero") point. This is a syntactic
+  // convenience to aid in code readability, mainly for debugging/testing use
+  // cases.
+  //
+  // Warning: While the Time subclass has a fixed origin point, the origin for
+  // the other subclasses can vary each time the application is restarted.
+  constexpr TimeDelta since_origin() const {
+    return TimeDelta::FromMicroseconds(us_);
+  }
+
   TimeClass& operator=(TimeClass other) {
     us_ = other.us_;
     return *(static_cast<TimeClass*>(this));

src/inspector/v8-profiler-agent-impl.cc

@@ -299,8 +299,7 @@ Response V8ProfilerAgentImpl::startPreciseCoverage(Maybe<bool> callCount,
                                                    double* out_timestamp) {
   if (!m_enabled) return Response::Error("Profiler is not enabled");
   *out_timestamp =
-      (v8::base::TimeTicks::HighResolutionNow() - v8::base::TimeTicks())
-          .InSecondsF();
+      v8::base::TimeTicks::HighResolutionNow().since_origin().InSecondsF();
   bool callCountValue = callCount.fromMaybe(false);
   bool detailedValue = detailed.fromMaybe(false);
   m_state->setBoolean(ProfilerAgentState::preciseCoverageStarted, true);
@@ -411,8 +410,7 @@ Response V8ProfilerAgentImpl::takePreciseCoverage(
   v8::HandleScope handle_scope(m_isolate);
   v8::debug::Coverage coverage = v8::debug::Coverage::CollectPrecise(m_isolate);
   *out_timestamp =
-      (v8::base::TimeTicks::HighResolutionNow() - v8::base::TimeTicks())
-          .InSecondsF();
+      v8::base::TimeTicks::HighResolutionNow().since_origin().InSecondsF();
   return coverageToProtocol(m_session->inspector(), coverage, out_result);
 }
 

src/profiler/profile-generator.cc

@@ -497,8 +497,7 @@ CpuProfile::CpuProfile(CpuProfiler* profiler, const char* title,
       streaming_next_sample_(0),
       id_(++last_id_) {
   auto value = TracedValue::Create();
-  value->SetDouble("startTime",
-                   (start_time_ - base::TimeTicks()).InMicroseconds());
+  value->SetDouble("startTime", start_time_.since_origin().InMicroseconds());
   TRACE_EVENT_SAMPLE_WITH_ID1(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profiler"),
                               "Profile", id_, "data", std::move(value));
 
@@ -639,7 +638,7 @@ void CpuProfile::FinishProfile() {
   context_filter_ = nullptr;
   StreamPendingTraceEvents();
   auto value = TracedValue::Create();
-  value->SetDouble("endTime", (end_time_ - base::TimeTicks()).InMicroseconds());
+  value->SetDouble("endTime", end_time_.since_origin().InMicroseconds());
   TRACE_EVENT_SAMPLE_WITH_ID1(TRACE_DISABLED_BY_DEFAULT("v8.cpu_profiler"),
                               "ProfileChunk", id_, "data", std::move(value));
 }