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/resource_bundle_handler_ctocpp.h

@@ -31,9 +31,11 @@ class CefResourceBundleHandlerCToCpp
   CefResourceBundleHandlerCToCpp();
 
   // CefResourceBundleHandler methods.
-  bool GetLocalizedString(int message_id, CefString& string) override;
+  bool GetLocalizedString(int string_id, CefString& string) override;
   bool GetDataResource(int resource_id, void*& data,
       size_t& data_size) override;
+  bool GetDataResourceForScale(int resource_id, ScaleFactor scale_factor,
+      void*& data, size_t& data_size) override;
 };
 
 #endif  // BUILDING_CEF_SHARED