ScriptController: limit script execution API to executeScript and callFunction, pass valid ScriptExecutionContext into ::callFunction.

BUG=297619

Review URL: https://chromiumcodereview.appspot.com/24758003

git-svn-id: svn://svn.chromium.org/blink/trunk@158437 bbb929c8-8fbe-4397-9dbb-9b2b20218538