[cc] Adds source_id and sequence_number to BeginFrameArgs.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 3b8ff9deccac7088e3aeba591d0f7bc9c2e6e301..8f5f7c0d10e6a5c1f6dd5ebf4a3d731a576bd479 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -46,6 +46,7 @@ class CC_EXPORT LayerTreeHost {
 
   // The current source frame number. This is incremented for each main frame
   // update(commit) pushed to the compositor thread.
+  // TODO(eseckler): Consider replacing this with BeginFrame sequence numbers.

[brianderson, 2016/12/15 22:10:33]

I think we can delete this TODO. SourceFrameNumber increments for every commit,
but BeginFrame sequence numbers increment for every BeginFrame sent.

A lot of tests rely on SourceFrameNumber to wait for a specific commit to
complete. Relying on BeginFrame numbers for that would not be reliable since
there could be multiple BeginFrames between commits.

[Eric Seckler, 2016/12/16 17:25:58]

Ok, sounds good! Thanks :)

   virtual int SourceFrameNumber() const = 0;
 
   // Returns the LayerTree that holds the main frame state pushed to the