Enable 2 pending frames on ARM devices.

content/browser/compositor/gpu_surfaceless_browser_compositor_output_surface.cc

 // Copyright 2014 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 "content/browser/compositor/gpu_surfaceless_browser_compositor_output_surface.h"
 
 #include "cc/output/compositor_frame.h"
 #include "content/browser/compositor/buffer_queue.h"
 #include "content/browser/compositor/reflector_impl.h"
 #include "content/browser/gpu/gpu_surface_tracker.h"
@@ skipping 12 matching lines @@
         scoped_ptr<cc::OverlayCandidateValidator> overlay_candidate_validator,
         unsigned internalformat,
         BrowserGpuMemoryBufferManager* gpu_memory_buffer_manager)
     : GpuBrowserCompositorOutputSurface(context,
                                         vsync_manager,
                                         overlay_candidate_validator.Pass()),
       internalformat_(internalformat),
       gpu_memory_buffer_manager_(gpu_memory_buffer_manager) {
   capabilities_.uses_default_gl_framebuffer = false;
   capabilities_.flipped_output_surface = true;
-  // TODO(alexst): Can't enable this on ARM since it hangs the device
-  // (crbug.com/470185). Alternatively remove this entirely once the compositor
-  // scheduling is optimized enough to work well with surfaceless.
-#if defined(ARCH_CPU_X86_FAMILY)
+  // Set |max_frames_pending| to 2 for surfaceless, which aligns scheduling
+  // more closely with the previous surfaced behavior.
+  // With a surface, swap buffer ack used to return early, before actually
+  // presenting the back buffer, enabling the browser compositor to run ahead.
+  // Surfaceless implementation acks at the time of actual buffer swap, which
+  // shifts the start of the new frame forward relative to the old
+  // implementation.
   capabilities_.max_frames_pending = 2;
-#endif
 
   gl_helper_.reset(new GLHelper(context_provider_->ContextGL(),
                                 context_provider_->ContextSupport()));
   output_surface_.reset(
       new BufferQueue(context_provider_, internalformat_, gl_helper_.get(),
                       gpu_memory_buffer_manager_, surface_id));
   output_surface_->Initialize();
 }
 
 GpuSurfacelessBrowserCompositorOutputSurface::
@@ skipping 35 matching lines @@
 
 void GpuSurfacelessBrowserCompositorOutputSurface::Reshape(
     const gfx::Size& size,
     float scale_factor) {
   GpuBrowserCompositorOutputSurface::Reshape(size, scale_factor);
   DCHECK(output_surface_);
   output_surface_->Reshape(SurfaceSize(), scale_factor);
 }
 
 }  // namespace content