cc: Don't swap throttle BeginMainFrame just after swapping

cc/scheduler/scheduler_state_machine.cc

Index: cc/scheduler/scheduler_state_machine.cc
diff --git a/cc/scheduler/scheduler_state_machine.cc b/cc/scheduler/scheduler_state_machine.cc
index f7919a6a36e0e3ff094ec12a6f1da8e2642f86a8..dea25b40871b440405ed48ea51300224770c26cb 100644
--- a/cc/scheduler/scheduler_state_machine.cc
+++ b/cc/scheduler/scheduler_state_machine.cc
@@ -464,9 +464,12 @@ bool SchedulerStateMachine::ShouldSendBeginMainFrame() const {
   if (!HasInitializedOutputSurface())
     return false;
 
-  // SwapAck throttle the BeginMainFrames
+  // SwapAck throttle the BeginMainFrames unless we just swapped.

[danakj, 2014/06/11 23:08:57]

This sounds like the opposite of what you have below, more like "unless we
didn't swap inside this deadline yet".

But then, doesn't that make the concept of pending swaps kinda pointless here?
We never wanna commit more than once in a deadline, and this would less us
commit every impl deadline?

[brianderson, 2014/06/11 23:25:49]

I think the comment and logic do match, it's just a little confusing. I actually
got the logic wrong at first until I updated the unit test.
It disables the swap throttling only immediately after a swap within the same
deadline. All subsequent deadlines will not send BeginMainFrames if we remain
swap throttled.

[Sami, 2014/06/13 10:39:12]

The conceptual double negatives here make your head spin don't they :) The logic
below looks correct to me, but I really hope we can make this a little less
ad-hoc in the future. Maybe it's time to consider not throttling at all here, or
even getting rid of smoothness mode.

[brianderson, 2014/06/13 18:19:29]

I've been trying to figure out why getting rid of this restriction causes
browser test flakiness in
https://code.google.com/p/chromium/issues/detail?id=368116. I really really want
to get rid of this check.

   // TODO(brianderson): Remove this restriction to improve throughput.
-  if (pending_swaps_ >= max_pending_swaps_)
+  bool just_swapped_in_deadline =
+      begin_impl_frame_state_ == BEGIN_IMPL_FRAME_STATE_INSIDE_DEADLINE &&
+      HasSwappedThisFrame();
+  if (pending_swaps_ >= max_pending_swaps_ && !just_swapped_in_deadline)
     return false;
 
   if (skip_begin_main_frame_to_reduce_latency_)