Remove TreatNullAs=NullString for HTMLImageElement

http://whatwg.org/html#htmlimageelement
http://whatwg.org/html#HTMLImageElement-partial

Take the opportunity to move standardized attributes into the main
block and sort it.

The border attribute has [TreatNullAs=EmptyString] in the spec.

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

Firefox and Opera do not reflect lowsrc as a URL, and do not support
srcset.

IE throws an exception for align and does not support srcset.

BUG=310298

Review URL: https://codereview.chromium.org/98833002

git-svn-id: svn://svn.chromium.org/blink/trunk@162990 bbb929c8-8fbe-4397-9dbb-9b2b20218538