cc: Clean up max frames/swaps pending usage.

cc/trees/layer_tree_host_impl.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/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <map>
 #include <set>
@@ skipping 2282 matching lines @@
 
   // TODO(brianderson): Don't use a hard-coded parent draw time.
   base::TimeDelta parent_draw_time =
       (!settings_.use_external_begin_frame_source &&
        output_surface_->capabilities().adjust_deadline_for_parent)
           ? BeginFrameArgs::DefaultEstimatedParentDrawTime()
           : base::TimeDelta();
   client_->SetEstimatedParentDrawTime(parent_draw_time);
 
   int max_frames_pending = output_surface_->capabilities().max_frames_pending;
-  if (max_frames_pending <= 0)
-    max_frames_pending = OutputSurface::DEFAULT_MAX_FRAMES_PENDING;
+  DCHECK_EQ(1, max_frames_pending);

[sunnyps, 2015/11/16 20:17:46]

Why not remove the max_frames_pending capability completely? Keeping it has
potential to cause confusion about whether we're supposed to support values
other than 1.

[brianderson, 2015/11/16 20:56:58]

I guess we could remove it from LTHI and cc::Scheduler. I'll do that as part of
this patch.

The OutputSurface for the DisplayScheduler can have more than 1 frame pending
though (for example on Chrome OS) - so we'll still need to keep it in the
OutputSurface::Capability.

   client_->SetMaxSwapsPendingOnImplThread(max_frames_pending);
   client_->OnCanDrawStateChanged(CanDraw());
 
   // There will not be anything to draw here, so set high res
   // to avoid checkerboards, typically when we are recovering
   // from lost context.
   // TODO(crbug.com/469175): Replace with RequiresHighResToDraw.
   SetRequiresHighResToDraw();
 
   return true;
@@ skipping 1379 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc