Make it possible for downstream projects to easily set the default fill color

content/browser/renderer_host/render_widget_host_view_mac.mm

 // 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 "content/browser/renderer_host/render_widget_host_view_mac.h"
 
 #import <objc/runtime.h>
 #include <OpenGL/gl.h>
 #include <QuartzCore/QuartzCore.h>
 #include <stdint.h>
@@ skipping 507 matching lines @@
   // namespace for surface-based hit testing.
   if (render_widget_host_->delegate() &&
       render_widget_host_->delegate()->GetInputEventRouter()) {
     render_widget_host_->delegate()
         ->GetInputEventRouter()
         ->AddSurfaceClientIdOwner(GetSurfaceClientId(), this);
   }
 
   RenderViewHost* rvh = RenderViewHost::From(render_widget_host_);
   if (rvh) {
-    // TODO(mostynb): actually use prefs.  Landing this as a separate CL
-    // first to rebaseline some unreliable layout tests.
-    ignore_result(rvh->GetWebkitPreferences());
+    WebPreferences prefs = rvh->GetWebkitPreferences();
+    background_color_ = prefs.default_background_color;
+    render_widget_host_->SetBackgroundOpaque(GetBackgroundOpaque());
   }
 }
 
 RenderWidgetHostViewMac::~RenderWidgetHostViewMac() {
   display::Screen::GetScreen()->RemoveObserver(this);
 
   // This is being called from |cocoa_view_|'s destructor, so invalidate the
   // pointer.
   cocoa_view_ = nil;
 
@@ skipping 2694 matching lines @@
 
 // "-webkit-app-region: drag | no-drag" is implemented on Mac by excluding
 // regions that are not draggable. (See ControlRegionView in
 // native_app_window_cocoa.mm). This requires the render host view to be
 // draggable by default.
 - (BOOL)mouseDownCanMoveWindow {
   return YES;
 }
 
 @end