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/common/main_delegate.cc

@@ -27,10 +27,12 @@
 #include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/widevine_cdm_constants.h"
+#include "components/content_settings/core/common/content_settings_pattern.h"
 #include "content/public/browser/browser_main_runner.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/main_function_params.h"
+#include "extensions/common/constants.h"
 #include "pdf/pdf.h"
 #include "ui/base/layout.h"
 #include "ui/base/resource/resource_bundle.h"
@@ -483,6 +485,9 @@ bool CefMainDelegate::BasicStartupComplete(int* exit_code) {
 
   logging::InitLogging(log_settings);
 
+  ContentSettingsPattern::SetNonWildcardDomainNonPortScheme(
+      extensions::kExtensionScheme);
+
   content::SetContentClient(&content_client_);
 
   return false;