Add a ScriptForbiddenScope to SVGImageChromeClient

The comment in ::animationTimerFired was wrong, the PageAnimator's
serviceScriptedAnimations does run requestAnimationFrame callbacks, but
SVGImages never have any so lets just add a ScriptForbiddenScope.

Review URL: https://codereview.chromium.org/265053004

git-svn-id: svn://svn.chromium.org/blink/trunk@173251 bbb929c8-8fbe-4397-9dbb-9b2b20218538