Add iterator object to iterate efficiently over an Element's attributes

Add iterator object to iterate efficiently over an Element's attributes instead
of using ElementData::length() / ElementData::attributeItem(index).

This is a bit more efficient as it avoids implicitly calling
ElementData::isUnique() for each loop iteration.

This is based on the following WebKit revision by benjamin@webkit.org:
http://trac.webkit.org/changeset/162394

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/298253009

git-svn-id: svn://svn.chromium.org/blink/trunk@175583 bbb929c8-8fbe-4397-9dbb-9b2b20218538