Send idle notifications to V8 after commit

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index d21b85bcc42af66ba2ba9fe3263bac6ca0ce3aad..777d5166997a6f3a8122c112d09f9406e7ed91dd 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -38,6 +38,7 @@
 #include "third_party/WebKit/public/platform/WebCompositeAndReadbackAsyncCallback.h"
 #include "third_party/WebKit/public/platform/WebSelectionBound.h"
 #include "third_party/WebKit/public/platform/WebSize.h"
+#include "third_party/WebKit/public/web/WebKit.h"
 #include "third_party/WebKit/public/web/WebWidget.h"
 #include "ui/gfx/frame_time.h"
 #include "ui/gl/gl_switches.h"
@@ -757,6 +758,7 @@ void RenderWidgetCompositor::DidBeginMainFrame() {
 }
 
 void RenderWidgetCompositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
+  begin_main_frame_args_ = args;
   double frame_time = (args.frame_time - base::TimeTicks()).InSecondsF();
   WebBeginFrameArgs web_begin_frame_args = WebBeginFrameArgs(frame_time);
   widget_->webwidget()->beginFrame(web_begin_frame_args);
@@ -785,6 +787,16 @@ void RenderWidgetCompositor::WillCommit() {
 }
 
 void RenderWidgetCompositor::DidCommit() {
+  CommandLine* cmd = CommandLine::ForCurrentProcess();
+  if (cmd->HasSwitch(switches::kSendV8IdleNotificationAfterCommit)) {

[jamesr, 2014/08/20 19:38:41]

grab this value once and stash it in a bool, it can't change over time. 
HasSwitch() is a lowercase conversion (on windows) and an std::map<> lookup, so
not terribly slow, but no need to bother doing this over and over when it'll
always give us the same answer

[ernstm, 2014/08/21 18:22:00]

Done.

+    int idle_time_in_ms =
+        std::max(0L, (begin_main_frame_args_.frame_time +
+                      begin_main_frame_args_.interval -
+                      base::TimeTicks::HighResNow()).InMilliseconds());

[jamesr, 2014/08/20 19:38:41]

i was under the impression BeginFrameArgs was built on top of
gfx::FrameTime::Now(), not base::TimeTicks::HighResNow().  Can you check this
please?

[brianderson, 2014/08/20 20:07:47]

James is correct. Using HighResNow() here directly will break on Windows. You
should use gfx::FrameTime::Now() instead.

[ernstm, 2014/08/21 18:22:00]

Done.

+    if (idle_time_in_ms)
+      blink::mainThreadIsolate()->IdleNotification(idle_time_in_ms);

[jamesr, 2014/08/20 19:38:41]

why the main thread isolate in particular? what about other heaps?

[ernstm, 2014/08/21 18:22:00]

The goal is to reduce jank caused by GC on the main thread before the commit.
Web workers already run in separate threads, so shouldn't cause problems.

+  }
+
   widget_->DidCommitCompositorFrame();
   widget_->didBecomeReadyForAdditionalInput();
 }