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/request_context_handler_ctocpp.h

@@ -32,6 +32,10 @@ class CefRequestContextHandlerCToCpp
 
   // CefRequestContextHandler methods.
   CefRefPtr<CefCookieManager> GetCookieManager() override;
+  bool OnBeforePluginLoad(const CefString& mime_type,
+      const CefString& plugin_url, const CefString& top_origin_url,
+      CefRefPtr<CefWebPluginInfo> plugin_info,
+      PluginPolicy* plugin_policy) override;
 };
 
 #endif  // BUILDING_CEF_SHARED