Remove TreatNullAs=NullString for HTMLElement

http://whatwg.org/html#htmlelement

Leave [TreatNullAs=NullString] on innerText/outerText, since they're
non-standard properties which need special investigation.

Split the tests for Element into a separate block for clarity.

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

Firefox reflects dir as "" and innerText as null.

IE throws an exception for dir and reflects innerText as "null".

No tested browser throws an exception when setting outerHTML/outerText
to null, but ignore that since the behavior wasn't change here.

BUG=310298

Review URL: https://codereview.chromium.org/102763002

git-svn-id: svn://svn.chromium.org/blink/trunk@163116 bbb929c8-8fbe-4397-9dbb-9b2b20218538