Add a flag to disable partial raster in renderer

ui/compositor/compositor.cc

 // Copyright (c) 2012 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 "ui/compositor/compositor.h"
 
 #include <algorithm>
 #include <deque>
 
 #include "base/bind.h"
@@ skipping 96 matching lines @@
     }
   }
   settings.renderer_settings.partial_swap_enabled =
       !command_line->HasSwitch(cc::switches::kUIDisablePartialSwap);
 #if defined(OS_WIN)
   settings.renderer_settings.finish_rendering_on_resize = true;
 #elif defined(OS_MACOSX)
   settings.renderer_settings.delay_releasing_overlay_resources = true;
 #endif
 
+  // Partial swap is always enabled for UI.
+  settings.enable_partial_raster = true;
+
   // These flags should be mirrored by renderer versions in content/renderer/.
   settings.initial_debug_state.show_debug_borders =
       command_line->HasSwitch(cc::switches::kUIShowCompositedLayerBorders);
   settings.initial_debug_state.show_fps_counter =
       command_line->HasSwitch(cc::switches::kUIShowFPSCounter);
   settings.initial_debug_state.show_layer_animation_bounds_rects =
       command_line->HasSwitch(cc::switches::kUIShowLayerAnimationBounds);
   settings.initial_debug_state.show_paint_rects =
       command_line->HasSwitch(switches::kUIShowPaintRects);
   settings.initial_debug_state.show_property_changed_rects =
@@ skipping 338 matching lines @@
                     observer_list_,
                     OnCompositingLockStateChanged(this));
 }
 
 void Compositor::CancelCompositorLock() {
   if (compositor_lock_)
     compositor_lock_->CancelLock();
 }
 
 }  // namespace ui