[heap] Add UMA histograms for background GC work.

This adds the following histograms:
- V8.GCBackgroundMarking
- V8.GCBackgroundScavenger
- V8.GCBackgroundSweeping

Bug: chromium:792552
Change-Id: Iae6fa3258f4fe0d4ed5e415c541a6d29101893a9
Reviewed-on: https://chromium-review.googlesource.com/819530Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50005}

src/counters.h

@@ -1043,6 +1043,9 @@ class RuntimeCallTimerScope {
 
 #define HISTOGRAM_RANGE_LIST(HR)                                               \
   /* Generic range histograms: HR(name, caption, min, max, num_buckets) */     \
+  HR(background_marking, V8.GCBackgroundMarking, 0, 10000, 101)                \
+  HR(background_scavenger, V8.GCBackgroundScavenger, 0, 10000, 101)            \
+  HR(background_sweeping, V8.GCBackgroundSweeping, 0, 10000, 101)              \
   HR(detached_context_age_in_gc, V8.DetachedContextAgeInGC, 0, 20, 21)         \
   HR(code_cache_reject_reason, V8.CodeCacheRejectReason, 1, 6, 6)              \
   HR(errors_thrown_per_context, V8.ErrorsThrownPerContext, 0, 200, 20)         \

src/heap/gc-tracer.cc

@@ -968,6 +968,10 @@ void GCTracer::FetchBackgroundMarkCompactCounters() {
                           Scope::LAST_MC_BACKGROUND_SCOPE,
                           BackgroundScope::FIRST_MC_BACKGROUND_SCOPE,
                           BackgroundScope::LAST_MC_BACKGROUND_SCOPE);
+  heap_->isolate()->counters()->background_marking()->AddSample(
+      static_cast<int>(current_.scopes[Scope::MC_BACKGROUND_MARKING]));
+  heap_->isolate()->counters()->background_sweeping()->AddSample(
+      static_cast<int>(current_.scopes[Scope::MC_BACKGROUND_SWEEPING]));
 }
 
 void GCTracer::FetchBackgroundMinorGCCounters() {
@@ -975,6 +979,9 @@ void GCTracer::FetchBackgroundMinorGCCounters() {
                           Scope::LAST_MINOR_GC_BACKGROUND_SCOPE,
                           BackgroundScope::FIRST_MINOR_GC_BACKGROUND_SCOPE,
                           BackgroundScope::LAST_MINOR_GC_BACKGROUND_SCOPE);
+  heap_->isolate()->counters()->background_scavenger()->AddSample(
+      static_cast<int>(
+          current_.scopes[Scope::SCAVENGER_BACKGROUND_SCAVENGE_PARALLEL]));
 }
 
 void GCTracer::FetchBackgroundGeneralCounters() {