cc/blimp: Add synchronization for scroll/scale state.

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 30de9244e1f9f394e5109c14b6bb4f78b37dfbd6..fdf78573e7afe77d9eac862959c4e8a7da604853 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -341,6 +341,11 @@ void LayerTreeHostImpl::BeginCommit() {
 void LayerTreeHostImpl::CommitComplete() {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::CommitComplete");
 
+  // If the state was committed to the pending tree, apply any reflected deltas

[danakj, 2016/10/25 22:14:43]

sync tree

[Khushal, 2016/10/26 01:03:29]

Done.

+  // that were not applied on the main thread.

[danakj, 2016/10/25 22:14:43]

applied during the main frame

[Khushal, 2016/10/26 01:03:29]

Done.

+  if (pending_tree())
+    pending_tree()->ApplyReflectedMainFrameState();

[danakj, 2016/10/25 22:14:43]

Shouldn't it do this on the sync tree always?

[Khushal, 2016/10/25 22:44:33]

Hmmm, if I copy to the pending base here, I don't think it will work. Looking at
the example of page scale, we PushFromMainThread during commit, and when
committing to the active tree, PushPendingToActive in the same call. And since
this happens post commit, the value will never make it to the active tree in
that mode.

I could probably check in LayerTreeImpl when over-writing the value. If its the
active tree, add the delta to the active base, otherwise the pending base. How
does that sound?

[danakj, 2016/10/25 23:55:55]

That sounds ok I think. If this sounds like complexity you won't use then maybe
this condition should be (!CommitToActive()) to show that intent more clearly
and DCHECK that otherwise its always empty unapplied stuff or something.

[Khushal, 2016/10/26 01:03:29]

Its not that complex to do, but we won't use it. So I avoided it. Added some
DCHECKs to make sure only the pending tree gets this state.

+
   if (CommitToActiveTree()) {
     // We have to activate animations here or "IsActive()" is true on the layers
     // but the animations aren't activated yet so they get ignored by