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.

tests/unittests/client_app_delegates.cc

@@ -19,6 +19,11 @@ void CreateBrowserDelegates(ClientAppBrowser::DelegateSet& delegates) {
       ClientAppBrowser::DelegateSet& delegates);
   CreateNavigationBrowserTests(delegates);
 
+  // Bring in the plugin tests.
+  extern void CreatePluginBrowserTests(
+      ClientAppBrowser::DelegateSet& delegates);
+  CreatePluginBrowserTests(delegates);
+
   // Bring in the RequestHandler tests.
   extern void CreateRequestHandlerBrowserTests(
       ClientAppBrowser::DelegateSet& delegates);