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_dll/ctocpp/context_menu_handler_ctocpp.h

@@ -34,6 +34,9 @@ class CefContextMenuHandlerCToCpp
   void OnBeforeContextMenu(CefRefPtr<CefBrowser> browser,
       CefRefPtr<CefFrame> frame, CefRefPtr<CefContextMenuParams> params,
       CefRefPtr<CefMenuModel> model) override;
+  bool RunContextMenu(CefRefPtr<CefBrowser> browser, CefRefPtr<CefFrame> frame,
+      CefRefPtr<CefContextMenuParams> params, CefRefPtr<CefMenuModel> model,
+      CefRefPtr<CefRunContextMenuCallback> callback) override;
   bool OnContextMenuCommand(CefRefPtr<CefBrowser> browser,
       CefRefPtr<CefFrame> frame, CefRefPtr<CefContextMenuParams> params,
       int command_id, EventFlags event_flags) override;