Support single-threaded impl-side painting

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 3b49d4c7a9e1ef16b2e3ffdb958b036a9a1c320e..3952794173f7ddf425a37873fe459090c0dbf8e7 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -277,6 +277,13 @@ scoped_ptr<RenderWidgetCompositor> RenderWidgetCompositor::Create(
       !cmd->HasSwitch(cc::switches::kDisableCompositedAntialiasing);
   settings.single_thread_proxy_scheduler =
       !cmd->HasSwitch(switches::kDisableSingleThreadProxyScheduler);
+  if (!threaded && !settings.single_thread_proxy_scheduler &&
+      settings.impl_side_painting) {
+    // The synchronous composite path requires zero copy with impl-side
+    // painting.
+    settings.use_zero_copy = true;
+    settings.scheduled_raster_task_limit = std::numeric_limits<size_t>::max();
+  }
 
   // These flags should be mirrored by UI versions in ui/compositor/.
   settings.initial_debug_state.show_debug_borders =