cc: Don't draw and swap if the frame will not change.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 264c11f098a84248ce2cf74a7482083fddd9f478..79b7179222987e8afc76c5185ce713625fa3734a 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -886,24 +886,30 @@ ThreadProxy::ScheduledActionDrawAndSwapInternal(bool forced_draw) {
 
   // This method is called on a forced draw, regardless of whether we are able
   // to produce a frame, as the calling site on main thread is blocked until its
-  // request completes, and we signal completion here. If canDraw() is false, we
+  // request completes, and we signal completion here. If CanDraw() is false, we
   // will indicate success=false to the caller, but we must still signal
   // completion to avoid deadlock.
 
-  // We guard prepareToDraw() with canDraw() because it always returns a valid
+  // We guard PrepareToDraw() with CanDraw() because it always returns a valid
   // frame, so can only be used when such a frame is possible. Since
-  // drawLayers() depends on the result of prepareToDraw(), it is guarded on
-  // canDraw() as well.
+  // DrawLayers() depends on the result of PrepareToDraw(), it is guarded on
+  // CanDraw() as well.
+
+  // If it is a forced draw, make sure we do a draw and swap.
+  gfx::Rect readback_rect;
+  if (readback_request_on_impl_thread_)
+    readback_rect = readback_request_on_impl_thread_->rect;
 
   LayerTreeHostImpl::FrameData frame;
   bool draw_frame =
       layer_tree_host_impl_->CanDraw() &&
-      (layer_tree_host_impl_->PrepareToDraw(&frame) || forced_draw);
+      (layer_tree_host_impl_->PrepareToDraw(&frame, readback_rect) ||
+       forced_draw);
   if (draw_frame) {
     layer_tree_host_impl_->DrawLayers(
         &frame,
         scheduler_on_impl_thread_->LastVSyncTime());
-    result.did_draw= true;
+    result.did_draw = true;
   }
   layer_tree_host_impl_->DidDrawAllLayers(frame);
 
@@ -928,7 +934,7 @@ ThreadProxy::ScheduledActionDrawAndSwapInternal(bool forced_draw) {
     readback_request_on_impl_thread_->completion.Signal();
     readback_request_on_impl_thread_ = NULL;
   } else if (draw_frame) {
-    result.did_swap = layer_tree_host_impl_->SwapBuffers();
+    result.did_swap = layer_tree_host_impl_->SwapBuffers(frame);
 
     if (frame.contains_incomplete_tile)
       DidSwapUseIncompleteTileOnImplThread();