Cache the current WebPreferences on RenderViewHostImpl.

content/public/browser/render_view_host.h

Index: content/public/browser/render_view_host.h
diff --git a/content/public/browser/render_view_host.h b/content/public/browser/render_view_host.h
index 840be8cb0667364800fa7e94129742a2f4207daa..2c4c25f59fe7de3a80006eb71b7895323578e096 100644
--- a/content/public/browser/render_view_host.h
+++ b/content/public/browser/render_view_host.h
@@ -194,9 +194,14 @@ class CONTENT_EXPORT RenderViewHost : virtual public RenderWidgetHost {
   // RenderViewHostDelegate.
   virtual void SyncRendererPrefs() = 0;
 
-  // Returns the current WebKit preferences.
+  // Returns the current WebKit preferences. Note: WebPreferences is cached, so
+  // this lookup will be fast
   virtual WebPreferences GetWebkitPreferences() = 0;
 
+  // If any state that affects the webkit preferences changed, this method must
+  // be called. This triggers recomputing preferences.
+  virtual void OnWebkitPreferencesChanged() = 0;
+
   // Passes a list of Webkit preferences to the renderer.
   virtual void UpdateWebkitPreferences(const WebPreferences& prefs) = 0;