ScriptController: clean up executeScript* API in order to identify call sites that require sync return value.

Drive-by: inline UserGestureIndicator usage.

Review URL: https://chromiumcodereview.appspot.com/23654057

git-svn-id: svn://svn.chromium.org/blink/trunk@158366 bbb929c8-8fbe-4397-9dbb-9b2b20218538