Support IDL attributes limited to only known values.

Provide the required binding layer support for IDL attributes that
reflect an enumerated content attribute that is limited to a set of
known values:

 http://www.whatwg.org/specs/web-apps/current-work/#limited-to-only-known-values

That is, have the getter for such attributes check with respect to the
known value set and perform the required filtering + canonicalization
of the value returned.

R=haraken
BUG=331695

Review URL: https://codereview.chromium.org/124763002

git-svn-id: svn://svn.chromium.org/blink/trunk@164509 bbb929c8-8fbe-4397-9dbb-9b2b20218538