Remove TreatNullAs=NullString for <base>, <body> and <br>

http://whatwg.org/html#htmlbaseelement
http://whatwg.org/html#HTMLBodyElement-partial
http://whatwg.org/html#HTMLBRElement-partial

Note that the spec has [TreatNullAs=EmptyString] for all HTMLBodyElement
attributes except background.

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

IE11 throws an exception when setting HTMLBRElement.clear,
which the spec does not allow for.

Presto resolves HTMLBodyElement.background as a URL.

BUG=310298

Review URL: https://codereview.chromium.org/60153008

git-svn-id: svn://svn.chromium.org/blink/trunk@161451 bbb929c8-8fbe-4397-9dbb-9b2b20218538