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();
+  // The means the renderer compositor has 2 possible modes:
+  // - Threaded compositing with a scheduler.
+  // - Single threaded compositing without a scheduler (for layout tests only).
+  // Using the scheduler in layout tests introduces additional composite steps
+  // that create flakiness.
+  settings.single_thread_proxy_scheduler = false;
 
   // 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