cc: Throttle swaps in Scheduler instead of OutputSurface

cc/scheduler/scheduler_state_machine.h

Index: cc/scheduler/scheduler_state_machine.h
diff --git a/cc/scheduler/scheduler_state_machine.h b/cc/scheduler/scheduler_state_machine.h
index c8d68fc429dbc2b4aa1b02211a4d8394e5f5cf5d..f7f48be1b6aa16b17a462609ba5ad7f426225029 100644
--- a/cc/scheduler/scheduler_state_machine.h
+++ b/cc/scheduler/scheduler_state_machine.h
@@ -168,10 +168,21 @@ class CC_EXPORT SchedulerStateMachine {
   // ManageTiles will occur shortly (even if no redraw is required).
   void SetNeedsManageTiles();
 
+  // Sets how many swaps can be pending to the OutputSurface.
+  void SetMaxSwapsPending(int max);
+
+  // If the scheduler attempted to draw and swap, this provides feedback
+  // regarding whether or not the swap actually occured. We might skip the

[danakj, 2014/04/11 15:12:28]

This overlaps with the DrawSwapReadbackResult::did_swap. Are you planning to
remove one of them? Why do we need both?

[brianderson, 2014/04/11 20:49:40]

Hmm, I was under the impression DrawSwapReadbackResult::did_swap would be true
even if there was no damage and we didn't end up swapping. That's only true for
the did_swap in SchedulerStateMachine::UpdateStateOnDraw(bool did_swap) though.

DidSwapBuffers is used for OutputSurface throttling and reflects whether or not
we actually swapped and should expect a swap ack. I'm not sure if there are any
other early outs other than the damage. I guess I early out on is_lost_, so
that's one difference in this patch.

How about I rename the did_swaps to did_request_swap?

[danakj, 2014/04/14 18:14:50]

The did_swap result is being dropped right now in
Scheduler::DrawAndSwapIfPossible() and Scheduler::DrawAndSwapForced().

What if those methods called back to the scheduler with the full
DrawSwapReadbackResult?

Right now the did_swap value seems only useful for the
Scheduler::DrawAndReadback() DCHECK on it.

Since we already have this data, it'd be nice to reuse it, or base the DCHECK on
the new mechanism and remove did_swap entirely?

+  // swap when there is not damage, for example.
+  void DidSwapBuffers();
+
   // Indicates whether a redraw is required because we are currently rendering
   // with a low resolution or checkerboarded tile.
   void SetSwapUsedIncompleteTile(bool used_incomplete_tile);
 
+  // Notification from the OutputSurface that a swap has been consumed.
+  void DidSwapBuffersComplete();
+
   // Indicates whether to prioritize animation smoothness over new content
   // activation.
   void SetSmoothnessTakesPriority(bool smoothness_takes_priority);
@@ -285,6 +296,8 @@ class CC_EXPORT SchedulerStateMachine {
   // ManageTile per BeginImplFrame.
   int manage_tiles_funnel_;
   int consecutive_checkerboard_animations_;
+  int max_pending_swaps_;
+  int pending_swaps_;
   bool needs_redraw_;
   bool needs_manage_tiles_;
   bool swap_used_incomplete_tile_;