Simplify Attr by removing m_specified member variable and setter

It always returned true in Blink, and doesn't exist in modern DOM specs.
Freezing it to true seems pretty safe.

R=arv

Review URL: https://codereview.chromium.org/26601004

git-svn-id: svn://svn.chromium.org/blink/trunk@159247 bbb929c8-8fbe-4397-9dbb-9b2b20218538