Use [TreatNullAs=EmptyString] for HTMLInputElement.value

This is what the specification has. Previous behavior for setting the
value IDL attribute to null was effectively to reset the form control's
value to the default, which is wrong.

New behavior also matches that of Firefox and Opera/Presto.

BUG=391194

Review URL: https://codereview.chromium.org/372543002

git-svn-id: svn://svn.chromium.org/blink/trunk@177564 bbb929c8-8fbe-4397-9dbb-9b2b20218538