cc: Add scheduler support for invalidating content on impl thread.

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index afc2247ec02d44af1bcdc54b6f6d85d2168ff2a5..5fd4c92e8d953cdd316bb5d8296963e536618973 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -45,6 +45,7 @@ class SchedulerClient {
   virtual void ScheduledActionBeginCompositorFrameSinkCreation() = 0;
   virtual void ScheduledActionPrepareTiles() = 0;
   virtual void ScheduledActionInvalidateCompositorFrameSink() = 0;
+  virtual void ScheduledActionPerformImplSideInvalidation() = 0;

[sunnyps, 2017/02/14 21:40:18]

nit/bikeshed: ScheduledActionInvalidateActiveTree?

[Khushal, 2017/02/15 00:56:33]

Hmmm, given that in the default case we actually create a pending tree to
invalidate on the active tree, I don't know if this makes it any clearer...

   virtual void DidFinishImplFrame() = 0;
   virtual void SendBeginMainFrameNotExpectedSoon() = 0;
 
@@ -89,6 +90,15 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
 
   void SetNeedsPrepareTiles();
 
+  // Requests a pending tree should be created to invalidate content on the impl
+  // thread, after the current tree is activated, if any. If the request
+  // necessitates creating a pending tree only for impl-side invalidations, the
+  // |client_| is informed to perform this action using
+  // ScheduledActionRunImplSideInvalidation.
+  // If ScheduledActionCommit is performed, the impl-side invalidations should
+  // be merged with the main frame and the request is assumed to be completed.
+  void SetNeedsImplSideInvalidation();
+
   // Drawing should result in submitting a CompositorFrame to the
   // CompositorFrameSink and then calling this.
   void DidSubmitCompositorFrame();