Make 'crossOrigin' IDL attributes nullable

This affects the 'crossOrigin' IDL attributes in the interfaces
HTMLImageElement, HTMLLinkElement, HTMLMediaElement and HTMLScriptElement,
and is done to match the corresponding specification change:
  http://html5.org/r/8727

The effect is that on setting the IDL attribute to null, the content
attribute is removed (if present) instead of set to the string "null", and
on getting null is returned instead of the empty string if the content
attribute is not present.

BUG=409524

Review URL: https://codereview.chromium.org/529933002

git-svn-id: svn://svn.chromium.org/blink/trunk@181728 bbb929c8-8fbe-4397-9dbb-9b2b20218538