Hook up ui::Compositor to Display's BeginFrameSource

cc/trees/layer_tree_settings.h

Index: cc/trees/layer_tree_settings.h
diff --git a/cc/trees/layer_tree_settings.h b/cc/trees/layer_tree_settings.h
index 2aadbbc90cb4d71c58ea8dfccfebf7180c8186b0..4e4de617f31ab3090bd8441886becac937b0de1e 100644
--- a/cc/trees/layer_tree_settings.h
+++ b/cc/trees/layer_tree_settings.h
@@ -38,7 +38,10 @@ class CC_EXPORT LayerTreeSettings {
 
   RendererSettings renderer_settings;
   bool single_thread_proxy_scheduler;
+  // TODO(enne): Remove this after everything uses output surface begin frames.
   bool use_external_begin_frame_source;
+  // TODO(enne): Temporary staging for unified begin frame source work.
+  bool use_output_surface_begin_frame_source;

[enne (OOO), 2016/03/28 21:43:27]

In order to stage Android (and renderers) separately, I've put in a
LayerTreeSetting for whether or not the output surface's begin frame source
should be used.  If the setting is off, then
OutputSurfaceClient::SetBeginFrameSource is ignored.  If the setting is on, then
SingleThreadProxy ignores external/unthrottled/synthetic begin frame sources and
only uses the one set by the OutputSurface and is otherwise null.  That seems
like a reasonable way to stage this for different compositors and platforms, and
also to make sure that this is behaving in a "real" way rather than depending
(briefly) on a fake begin frame source that gets clobbered.

   bool main_frame_before_activation_enabled;
   bool using_synchronous_renderer_compositor;
   bool can_use_lcd_text;