Make ui::Snapshot asynchronous on Android, remove CompositeAndReadback.

content/browser/renderer_host/compositor_impl_android.cc

Index: content/browser/renderer_host/compositor_impl_android.cc
diff --git a/content/browser/renderer_host/compositor_impl_android.cc b/content/browser/renderer_host/compositor_impl_android.cc
index fcd0a7b3bdd8568029e12ab39c2e8e32659dfc97..46d56dc6450ad9b654be46bd586b8b3d990802c1 100644
--- a/content/browser/renderer_host/compositor_impl_android.cc
+++ b/content/browser/renderer_host/compositor_impl_android.cc
@@ -439,10 +439,7 @@ void CompositorImpl::SetHasTransparentBackground(bool flag) {
 }
 
 bool CompositorImpl::CompositeAndReadback(void *pixels, const gfx::Rect& rect) {
-  if (host_)
-    return host_->CompositeAndReadback(pixels, rect);
-  else
-    return false;
+  return false;
 }
 
 void CompositorImpl::SetNeedsComposite() {
@@ -623,6 +620,12 @@ void CompositorImpl::AttachLayerForReadback(scoped_refptr<cc::Layer> layer) {
   root_layer_->AddChild(layer);
 }
 
+void CompositorImpl::RequestCopyOfOutputOnRootLayer(
+    scoped_ptr<cc::CopyOutputRequest> request) {
+  if (root_layer_)

[no sievers, 2014/05/13 23:58:00]

nit: The root_layer_ is always there

[Ted C, 2014/05/14 01:13:18]

And if we don't remove this conditional, do we need to handle the false case by
signaling the request failed?

[danakj, 2014/05/14 14:33:20]

Oh good, done.
No, when the CopyOutputRequest is destroyed before reaching the compositor it
runs the callback with an empty CopyOutputResult.

SnapshotAsync::ScaleCopyOutputResult and SnapshotAsync::EncodeCopyOutputResult
both check for the empty result.

+    root_layer_->RequestCopyOfOutput(request.Pass());
+}
+
 void CompositorImpl::OnVSync(base::TimeTicks frame_time,
                              base::TimeDelta vsync_period) {
   vsync_period_ = vsync_period;