Enable 2 pending frames on ARM devices.

cc/scheduler/scheduler_state_machine.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/scheduler/scheduler_state_machine.h"
 
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/strings/stringprintf.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 405 matching lines @@
 
   // We need a new commit for the forced redraw. This honors the
   // single commit per interval because the result will be swapped to screen.
   if (forced_redraw_state_ == FORCED_REDRAW_STATE_WAITING_FOR_COMMIT)
     return true;
 
   // We shouldn't normally accept commits if there isn't an OutputSurface.
   if (!HasInitializedOutputSurface())
     return false;
 
-  // SwapAck throttle the BeginMainFrames unless we just swapped.
-  // TODO(brianderson): Remove this restriction to improve throughput.

[brianderson, 2015/04/03 22:44:01]

Beware that this aspect of the patch might introduce flakiness in browser tests
and could result in regressing other use cases where it might increase the CPU
fighting. We should definitely keep an eye on the perf dashboard for this.

Do you get any benefit from just the other half of this patch? If so it might be
worth splitting them up so you don't have to revert both parts.

[alexst (slow to review), 2015/04/04 00:20:02]

On intel, just the other part was enough, on ARM, this was also required.

Maybe we should take a more cautious approach and #ifdef it out only for freon
on arm? This would let us monitor it on a smaller scale and not  cause side
effects elsewhere.

That being said, I think you are right, I'll split these 2 parts to keep impact
limited.

-  bool just_swapped_in_deadline =
-      begin_impl_frame_state_ == BEGIN_IMPL_FRAME_STATE_INSIDE_DEADLINE &&
-      perform_swap_funnel_;
-  if (pending_swaps_ >= max_pending_swaps_ && !just_swapped_in_deadline)
-    return false;
-
   if (skip_begin_main_frame_to_reduce_latency_)
     return false;
 
   return true;
 }
 
 bool SchedulerStateMachine::ShouldCommit() const {
   if (commit_state_ != COMMIT_STATE_READY_TO_COMMIT)
     return false;
 
@@ skipping 636 matching lines @@
       static_cast<int>(begin_impl_frame_state_),
       static_cast<int>(commit_state_),
       has_pending_tree_ ? 'T' : 'F',
       pending_tree_is_ready_for_activation_ ? 'T' : 'F',
       active_tree_needs_first_draw_ ? 'T' : 'F',
       max_pending_swaps_,
       pending_swaps_);
 }
 
 }  // namespace cc