cc: Async readback.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index cb53f79fbe117bc7a5da8be196ced524ef31d472..5a0015ef8925706226bea7580b1db9de91a93c97 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -577,6 +577,9 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
   // due to an impl-animation, we drop the frame to avoid flashing due to the
   // texture suddenly appearing in the future.
   bool draw_frame = true;
+  // When we have a copy request for a layer, we need to draw no matter
+  // what, as the layer may disappear after this frame.
+  bool have_copy_request = false;
 
   int layers_drawn = 0;
 
@@ -595,7 +598,12 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
 
     AppendQuadsData append_quads_data(target_render_pass->id);
 
-    if (it.represents_contributing_render_surface()) {
+    if (it.represents_target_render_surface()) {
+      if (it->HasRequestCopyCallback()) {
+        have_copy_request = true;
+        it->TakeRequestCopyCallbacks(&target_render_pass->copy_callbacks);
+      }
+    } else if (it.represents_contributing_render_surface()) {
       RenderPass::Id contributing_render_pass_id =
           it->render_surface()->RenderPassId();
       RenderPass* contributing_render_pass =
@@ -671,6 +679,9 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
     occlusion_tracker.LeaveLayer(it);
   }
 
+  if (have_copy_request)
+    draw_frame = true;
+
   rendering_stats_instrumentation_->AddLayersDrawn(layers_drawn);
 
 #ifndef NDEBUG
@@ -693,6 +704,8 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
 
   if (draw_frame)
     occlusion_tracker.overdraw_metrics()->RecordMetrics(this);
+  else
+    DCHECK(!have_copy_request);
 
   RemoveRenderPasses(CullRenderPassesWithNoQuads(), frame);
   renderer_->DecideRenderPassAllocationsForFrame(frame->render_passes);