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/capi/cef_request_handler_capi.h

@@ -46,7 +46,6 @@
 #include "include/capi/cef_resource_handler_capi.h"
 #include "include/capi/cef_response_capi.h"
 #include "include/capi/cef_ssl_info_capi.h"
-#include "include/capi/cef_web_plugin_capi.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -214,14 +213,6 @@ typedef struct _cef_request_handler_t {
       const cef_string_t* request_url, struct _cef_sslinfo_t* ssl_info,
       struct _cef_request_callback_t* callback);
 
-  ///
-  // Called on the browser process IO thread before a plugin is loaded. Return
-  // true (1) to block loading of the plugin.
-  ///
-  int (CEF_CALLBACK *on_before_plugin_load)(struct _cef_request_handler_t* self,
-      struct _cef_browser_t* browser, const cef_string_t* url,
-      const cef_string_t* policy_url, struct _cef_web_plugin_info_t* info);
-
   ///
   // Called on the browser process UI thread when a plugin has crashed.
   // |plugin_path| is the path of the plugin that crashed.