cc: Make single-thread renderer compositor always synchronous.

content/renderer/gpu/render_widget_compositor.cc

 // Copyright (c) 2013 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/renderer/gpu/render_widget_compositor.h"
 
 #include <limits>
 #include <string>
 
 #include "base/command_line.h"
@@ skipping 297 matching lines @@
         settings.top_controls_hide_threshold = hide_threshold;
   }
 
   settings.use_pinch_virtual_viewport =
       cmd->HasSwitch(cc::switches::kEnablePinchVirtualViewport);
   settings.verify_property_trees =
       cmd->HasSwitch(cc::switches::kEnablePropertyTreeVerification) &&
       settings.impl_side_painting;
   settings.renderer_settings.allow_antialiasing &=
       !cmd->HasSwitch(cc::switches::kDisableCompositedAntialiasing);
-  settings.single_thread_proxy_scheduler =
-      compositor_deps_->UseSingleThreadScheduler();
+  // Using the scheduler in layout tests (the only time we're single threaded
+  // here) introduces additional composite steps that create flakiness.
+  settings.single_thread_proxy_scheduler = false;

[enne (OOO), 2015/05/20 19:05:23]

Can you leave a comment here that this means that the two states for the
renderer are multi-threaded+scheduler and single-threaded+synchronous, and that
this flag has no effect for threaded mode?

My worry here is somebody will see this and thinks this implies always using
single thread synchronous?

[danakj, 2015/05/20 19:07:38]

Done.

 
   // These flags should be mirrored by UI versions in ui/compositor/.
   settings.initial_debug_state.show_debug_borders =
       cmd->HasSwitch(cc::switches::kShowCompositedLayerBorders);
   settings.initial_debug_state.show_fps_counter =
       cmd->HasSwitch(cc::switches::kShowFPSCounter);
   settings.initial_debug_state.show_layer_animation_bounds_rects =
       cmd->HasSwitch(cc::switches::kShowLayerAnimationBounds);
   settings.initial_debug_state.show_paint_rects =
       cmd->HasSwitch(switches::kShowPaintRects);
@@ skipping 634 matching lines @@
     return;
   provider->ContextGL()->RateLimitOffscreenContextCHROMIUM();
 }
 
 void RenderWidgetCompositor::SetSurfaceIdNamespace(
     uint32_t surface_id_namespace) {
   layer_tree_host_->set_surface_id_namespace(surface_id_namespace);
 }
 
 }  // namespace content