bindings: Introduce a new IDL attribute, used only for TouchEvent.

All of the runtime feature-specific events (e.g. TrackEvent,
MediaKeyEvent etc.) can be created by JS (document.createEvent()) even
when the feature is disabled. TouchEvent is the only exception. So
introduce a new IDL attribute for touch-events, called
AllowJSCreationOnlyIfFeatureEnabled. When this attribute is specified for an
event, document.createEvent() will fail for the event unless the runtime-feature
has been enabled.

BUG=256230
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/18205010

git-svn-id: svn://svn.chromium.org/blink/trunk@154187 bbb929c8-8fbe-4397-9dbb-9b2b20218538