views: Add groups and focus/blur callbacks (issue #2102)

- cefclient: Only make menus focusable when ALT is pressed.
- cefclient: Display sample top menu when passed the `--show-top-menu`
  command-line flag.

tests/shared/common/client_switches.h

@@ -28,6 +28,7 @@ extern const char kFilterURL[];
 extern const char kUseViews[];
 extern const char kHideFrame[];
 extern const char kHideControls[];
+extern const char kShowTopMenu[];
 extern const char kWidevineCdmPath[];
 extern const char kSslClientCertificate[];