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.

include/cef_request_handler.h

@@ -46,7 +46,6 @@
 #include "include/cef_response.h"
 #include "include/cef_request.h"
 #include "include/cef_ssl_info.h"
-#include "include/cef_web_plugin.h"
 
 
 ///
@@ -248,18 +247,6 @@ class CefRequestHandler : public virtual CefBase {
     return false;
   }
 
-  ///
-  // Called on the browser process IO thread before a plugin is loaded. Return
-  // true to block loading of the plugin.
-  ///
-  /*--cef(optional_param=url,optional_param=policy_url)--*/
-  virtual bool OnBeforePluginLoad(CefRefPtr<CefBrowser> browser,
-                                  const CefString& url,
-                                  const CefString& policy_url,
-                                  CefRefPtr<CefWebPluginInfo> info) {
-    return false;
-  }
-
   ///
   // Called on the browser process UI thread when a plugin has crashed.
   // |plugin_path| is the path of the plugin that crashed.