WebView: Immediately ack and return resources for fallback ticks.

content/renderer/android/synchronous_compositor_output_surface.cc

Index: content/renderer/android/synchronous_compositor_output_surface.cc
diff --git a/content/renderer/android/synchronous_compositor_output_surface.cc b/content/renderer/android/synchronous_compositor_output_surface.cc
index d42d3de65465ee231350c2fd9707443b45a3fe9b..2d3b60921979154b2e7d7247a727c13ee04c2512 100644
--- a/content/renderer/android/synchronous_compositor_output_surface.cc
+++ b/content/renderer/android/synchronous_compositor_output_surface.cc
@@ -197,6 +197,17 @@ void SynchronousCompositorOutputSurface::SwapBuffers(
   DCHECK(CalledOnValidThread());
   DCHECK(sync_client_);
 
+  client_->DidSwapBuffers();

[boliu, 2016/07/07 21:30:31]

sync_client_->SwapBuffers sends the reply of the synchronous IPC to unblock the
UI thread, I don't know how expensive DidSwapBuffers, but probably better to not
have this here for regular draws.

+
+  if (fallback_tick_running_) {
+    client_->DidSwapBuffersComplete();
+    cc::ReturnedResourceArray return_resources;
+    cc::TransferableResource::ReturnResources(
+        frame.delegated_frame_data->resource_list, &return_resources);
+    ReturnResources(return_resources);

[boliu, 2016/07/07 21:30:31]

again, can we skip this and just DCHECK there are no resources?

[danakj, 2016/07/07 22:08:34]

DCHECKing empty, but still sending back the ack to the OutputSurface client.

+    return;
+  }
+
   cc::CompositorFrame swap_frame;
 
   if (in_software_draw_) {
@@ -220,14 +231,13 @@ void SynchronousCompositorOutputSurface::SwapBuffers(
         delegated_surface_id_, std::move(frame), base::Bind(&NoOpDrawCallback));
     display_->DrawAndSwap();
   } else {
+    // For hardware draws we send the whole frame to the client so it can draw
+    // the content in it.
     swap_frame = std::move(frame);
   }
 
-  if (!fallback_tick_running_) {
-    sync_client_->SwapBuffers(output_surface_id_, std::move(swap_frame));
-    DeliverMessages();
-  }
-  client_->DidSwapBuffers();
+  sync_client_->SwapBuffers(output_surface_id_, std::move(swap_frame));
+  DeliverMessages();
   did_swap_ = true;
 }