cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/blimp/layer_tree_host_remote.h

Index: cc/blimp/layer_tree_host_remote.h
diff --git a/cc/blimp/layer_tree_host_remote.h b/cc/blimp/layer_tree_host_remote.h
index e7c3c8b780e3cf4642c907a89bec2b64141f4efd..9eb61f320f6de3afda5d20fc6922228453655262 100644
--- a/cc/blimp/layer_tree_host_remote.h
+++ b/cc/blimp/layer_tree_host_remote.h
@@ -125,6 +125,7 @@ class CC_EXPORT LayerTreeHostRemote : public LayerTreeHost,
   int source_frame_number_ = 0;

[Sami, 2016/12/06 12:41:06]

aside: We might want to drop a TODO somewhere about replacing this with
begin_frame_number_ since I think it fills a similar use.

[Eric Seckler, 2016/12/06 17:33:59]

In general, the LayerTreeHost::SourceFrameNumber() seems to be incremented for
new main frames, rather than a BeginFrame. For LTHRemote that turns out to be
the same as the BeginFrame number used for its BeginFrameArgs creation, but
sign/width of the number are different - which is why I added another field. I'm
adding a TODO to the LTH interface to think about using BeginFrame numbers
instead of "source" frame numbers.

[Sami, 2016/12/07 16:59:39]

True, I was just thinking that since the only way to get a new main frame is via
a BeginFrame, so another counter doesn't really add any new information --
unless we want main frame numbers to be consecutive for some reason. In any case
I think we can defer this decision.

   bool visible_ = false;
   bool defer_commits_ = false;
+  uint64_t begin_frame_number_ = 0;
 
   // In threaded/single-threaded mode, the LayerTree and Layers expect scroll/
   // scale updates to come from the impl thread only during the main frame.