Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

libcef/browser/content_browser_client.h

@@ -32,7 +32,6 @@ class CefResourceDispatcherHostDelegate;
 class PrefService;
 
 namespace content {
-class PluginServiceFilter;
 class SiteInstance;
 }
 
@@ -174,7 +173,6 @@ class CefContentBrowserClient : public content::ContentBrowserClient {
 
   CefBrowserMainParts* browser_main_parts_;
 
-  scoped_ptr<content::PluginServiceFilter> plugin_service_filter_;
   scoped_ptr<CefResourceDispatcherHostDelegate>
       resource_dispatcher_host_delegate_;