Atomicize HTMLAnchorElement.hash before passing it to JS

Backport of WebKit patch by Andreas Kling <akling@apple.com>,
reviewed by Sam Weinig (http://webkit.org/b/118748)

Instead of generating a unique string from '#' + the URL hash every time
HTMLAnchorElement.hash is queried, turn it into an AtomicString. This
prevents massive memory usage in scripts that fetch .hash a lot.

Reduces memory consumption by 4.65 MB when viewing the full HTML5 spec at
<http://whatwg.org/c>

Review URL: https://chromiumcodereview.appspot.com/19677003

git-svn-id: svn://svn.chromium.org/blink/trunk@154447 bbb929c8-8fbe-4397-9dbb-9b2b20218538