Remove TreatNullAs=NullString for HTMLKeygenElement

http://whatwg.org/html#htmlkeygenelement

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

Firefox and IE support none of the attributes and thus fail all tests.

Opera reflects keytype as the empty string.

BUG=310298

Review URL: https://codereview.chromium.org/98543003

git-svn-id: svn://svn.chromium.org/blink/trunk@162995 bbb929c8-8fbe-4397-9dbb-9b2b20218538