[runtime-call-stats] Allow empty RCS stack upon Leave

The stack can be cleared with RuntimeCallStats::Reset() call.
Correctly handle the case by silently exit the running timer scopes.

BUG=chromium:760649

Change-Id: I51ecca5591a7af358f3e50779d0f81cb9d76e502
Reviewed-on: https://chromium-review.googlesource.com/734121Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48847}

src/counters.cc

@@ -479,7 +479,7 @@ void RuntimeCallStats::Enter(RuntimeCallStats* stats, RuntimeCallTimer* timer,
   CHECK(stats->IsCalledOnTheSameThread());
   RuntimeCallCounter* counter = &(stats->*counter_id);
   DCHECK_NOT_NULL(counter->name());
-  timer->Start(counter, stats->current_timer_.Value());
+  timer->Start(counter, stats->current_timer());
   stats->current_timer_.SetValue(timer);
   stats->current_counter_.SetValue(counter);
 }
@@ -487,17 +487,18 @@ void RuntimeCallStats::Enter(RuntimeCallStats* stats, RuntimeCallTimer* timer,
 // static
 void RuntimeCallStats::Leave(RuntimeCallStats* stats, RuntimeCallTimer* timer) {
   CHECK(stats->IsCalledOnTheSameThread());
-  if (stats->current_timer_.Value() != timer) {
+  RuntimeCallTimer* stack_top = stats->current_timer();
+  if (stack_top == nullptr) return;  // Missing timer is a result of Reset().
+  if (stack_top != timer) {
     // The branch is added to catch a crash crbug.com/760649
-    RuntimeCallTimer* stack_top = stats->current_timer_.Value();
     EmbeddedVector<char, 200> text;
     SNPrintF(text, "ERROR: Leaving counter '%s', stack top '%s'.\n",
-             timer->name(), stack_top ? stack_top->name() : "(null)");
+             timer->name(), stack_top->name());
     USE(text);
     CHECK(false);
   }
   stats->current_timer_.SetValue(timer->Stop());
-  RuntimeCallTimer* cur_timer = stats->current_timer_.Value();
+  RuntimeCallTimer* cur_timer = stats->current_timer();
   stats->current_counter_.SetValue(cur_timer ? cur_timer->counter() : nullptr);
 }