Use swap-promise to improve first* paint times

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 b6c7631c744cd0d7965191fd7105db39b4a6fbda..2adeb8b87e81ae0ba9c486d254b5305614c03901 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -186,8 +186,37 @@ static cc::BrowserControlsState ConvertBrowserControlsState(
   return static_cast<cc::BrowserControlsState>(state);
 }
 
+double MonotonicallyIncreasingTime() {
+  return base::TimeTicks::Now().ToInternalValue() /
+         static_cast<double>(base::Time::kMicrosecondsPerSecond);

[sunnyps, 2017/05/02 01:20:46]

nit: can you put the static_case<double> on the first part of the expression
(...ToInternalValue)?

[panicker, 2017/05/02 21:21:36]

Done.

+}
+
 }  // namespace
 
+RenderWidgetCompositor::ReportTimeSwapPromise::ReportTimeSwapPromise(
+    ReportTimeCallback callback,
+    const scoped_refptr<base::SingleThreadTaskRunner>& task_runner)
+    : callback_(callback), task_runner_(task_runner) {}
+
+RenderWidgetCompositor::ReportTimeSwapPromise::~ReportTimeSwapPromise() {}
+
+void RenderWidgetCompositor::ReportTimeSwapPromise::DidSwap() {
+  task_runner_->PostTask(
+      FROM_HERE,
+      base::BindOnce(callback_, true, MonotonicallyIncreasingTime()));
+}
+
+cc::SwapPromise::DidNotSwapAction
+RenderWidgetCompositor::ReportTimeSwapPromise::DidNotSwap(
+    cc::SwapPromise::DidNotSwapReason reason) {
+  task_runner_->PostTask(FROM_HERE, base::BindOnce(callback_, false, 0));
+  return cc::SwapPromise::DidNotSwapAction::BREAK_PROMISE;
+}
+
+int64_t RenderWidgetCompositor::ReportTimeSwapPromise::TraceId() const {
+  return 0;
+}
+
 // static
 std::unique_ptr<RenderWidgetCompositor> RenderWidgetCompositor::Create(
     RenderWidgetCompositorDelegate* delegate,
@@ -1155,4 +1184,10 @@ void RenderWidgetCompositor::SetLocalSurfaceId(
   layer_tree_host_->SetLocalSurfaceId(local_surface_id);
 }
 
+void RenderWidgetCompositor::NotifySwapTime(
+    base::Callback<void(bool, double)> callback) {
+  QueueSwapPromise(base::MakeUnique<ReportTimeSwapPromise>(
+      std::move(callback), base::ThreadTaskRunnerHandle::Get()));
+}
+
 }  // namespace content