Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/extensions/extensions_api_client.cc

@@ -13,7 +13,6 @@
 #include "libcef/browser/printing/print_view_manager.h"
 
 #include "base/memory/ptr_util.h"
-#include "chrome/browser/sessions/session_tab_helper.h"
 #include "chrome/browser/ui/prefs/prefs_tab_helper.h"
 #include "components/pdf/browser/pdf_web_contents_helper.h"
 #include "components/zoom/zoom_controller.h"
@@ -64,7 +63,6 @@ void CefExtensionsAPIClient::AttachWebContentsHelpers(
                         new CefPDFWebContentsHelperClient()));
 
   // Used by the tabs extension API.
-  SessionTabHelper::CreateForWebContents(web_contents);
   zoom::ZoomController::CreateForWebContents(web_contents);
 }