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/browser/extensions/extensions_browser_client.cc

@@ -5,7 +5,7 @@
 
 #include "libcef/browser/extensions/extensions_browser_client.h"
 
-#include "libcef/browser/content_browser_client.h"
+#include "libcef/browser/browser_context_impl.h"
 #include "libcef/browser/extensions/component_extension_resource_manager.h"
 #include "libcef/browser/extensions/extension_system_factory.h"
 #include "libcef/browser/extensions/extension_web_contents_observer.h"
@@ -124,8 +124,7 @@ bool CefExtensionsBrowserClient::AllowCrossRendererResourceLoad(
 
 PrefService* CefExtensionsBrowserClient::GetPrefServiceForContext(
     BrowserContext* context) {
-  // TODO(extensions): Do we need a per-context PrefService?
-  return CefContentBrowserClient::Get()->pref_service();
+  return CefBrowserContextImpl::GetForContext(context)->GetPrefs();
 }
 
 void CefExtensionsBrowserClient::GetEarlyExtensionPrefsObservers(