x11: Add a missing break statement from r283931.

BUG=none
TBR=kpschoedel@chromium.org

Review URL: https://codereview.chromium.org/403713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283987 0039d316-1c4b-4281-b951-d872f2087c98

ui/events/x/events_x.cc

@@ -601,6 +601,7 @@ uint32 PlatformKeycodeFromNative(const base::NativeEvent& native_event) {
           NOTREACHED();
           break;
       }
+      break;
     }
     default:
       NOTREACHED();