De-dupe copy requests for tab capture in DelegatedFrameHost.

content/browser/compositor/delegated_frame_host.cc

Index: content/browser/compositor/delegated_frame_host.cc
diff --git a/content/browser/compositor/delegated_frame_host.cc b/content/browser/compositor/delegated_frame_host.cc
index e6cb16150673df0c4db6cc80b28610e4154792a2..135961f051ad0b315084bc442078d907b1548c3c 100644
--- a/content/browser/compositor/delegated_frame_host.cc
+++ b/content/browser/compositor/delegated_frame_host.cc
@@ -64,6 +64,7 @@ DelegatedFrameHost::DelegatedFrameHost(DelegatedFrameHostClient* client)
       skipped_frames_(false),
       current_scale_factor_(1.f),
       can_lock_compositor_(YES_CAN_LOCK),
+      frame_subscriber_copy_request_pending_(false),
       delegated_frame_evictor_(new DelegatedFrameEvictor(this)) {
   ImageTransportFactory::GetInstance()->AddObserver(this);
 }
@@ -269,6 +270,18 @@ void DelegatedFrameHost::DidReceiveFrameFromRenderer(
   RenderWidgetHostViewFrameSubscriber::DeliverFrameCallback callback;
   if (frame_subscriber()->ShouldCaptureFrame(damage_rect, present_time,
                                              &frame, &callback)) {
+    if (frame_subscriber_copy_request_pending_) {
+      // A copy request was made for the previous frame from the renderer, but
+      // drawing never started (which executes the request).  Therefore, that
+      // request is still alive; and this request should be immediately aborted.
+      // Note that this de-duping check must be done after the call to
+      // ShouldCaptureFrame() above, since the frame subscriber makes decisions
+      // based on the renderer's intentions, and not the performance of the
+      // browser compositor.
+      callback.Run(present_time, false);
+      return;
+    }
+    frame_subscriber_copy_request_pending_ = true;
     CopyFromCompositingSurfaceToVideoFrame(
         gfx::Rect(current_frame_size_in_dip_),
         frame,
@@ -854,6 +867,7 @@ void DelegatedFrameHost::OnCompositingDidCommit(
 void DelegatedFrameHost::OnCompositingStarted(
     ui::Compositor* compositor, base::TimeTicks start_time) {
   last_draw_ended_ = start_time;
+  frame_subscriber_copy_request_pending_ = false;
 }
 
 void DelegatedFrameHost::OnCompositingEnded(