scheduler: Don't emit virtual time trace events unless virtual time is on

This patch makes WebScopedVirtualTimePauser avoid emitting trace events
if virtual time is disabled.

Bug: 1069603
Change-Id: I14bfe21bcd67f325018ffeec1d17e01fe2f1dde0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144143
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759273}

third_party/blink/public/platform/scheduler/web_scoped_virtual_time_pauser.h

@@ -60,6 +60,7 @@ class BLINK_PLATFORM_EXPORT WebScopedVirtualTimePauser {
 
   base::TimeTicks virtual_time_when_paused_;
   bool paused_ = false;
+  bool virtual_time_enabled_when_paused_ = false;
   VirtualTaskDuration duration_ = VirtualTaskDuration::kInstant;
   scheduler::MainThreadSchedulerImpl* scheduler_;  // NOT OWNED
   WebString debug_name_;

third_party/blink/renderer/platform/scheduler/main_thread/web_scoped_virtual_time_pauser.cc

@@ -57,9 +57,17 @@ void WebScopedVirtualTimePauser::PauseVirtualTime() {
     return;
 
   paused_ = true;
+  // Note that virtual time can never be disabled after it's enabled once, so we
+  // don't need to worry about the reverse transition.
+  virtual_time_enabled_when_paused_ = scheduler_->IsVirtualTimeEnabled();
+
+  if (virtual_time_enabled_when_paused_) {
+    // This trace event shows when individual pausers are active (instead of the
+    // global paused/unpaused state).
     TRACE_EVENT_NESTABLE_ASYNC_BEGIN1(
         "renderer.scheduler", "WebScopedVirtualTimePauser::PauseVirtualTime",
         trace_id_, "name", debug_name_.Latin1());
+  }
   virtual_time_when_paused_ = scheduler_->IncrementVirtualTimePauseCount();
 }
 
@@ -77,9 +85,11 @@ void WebScopedVirtualTimePauser::DecrementVirtualTimePauseCount() {
     scheduler_->MaybeAdvanceVirtualTime(virtual_time_when_paused_ +
                                         base::TimeDelta::FromMilliseconds(10));
   }
+  if (virtual_time_enabled_when_paused_) {
     TRACE_EVENT_NESTABLE_ASYNC_END0(
         "renderer.scheduler", "WebScopedVirtualTimePauser::PauseVirtualTime",
         trace_id_);
+  }
 }
 
 }  // namespace blink