Deprecate Attr.specified.

Use counters measure usage of Attr.specified at 0.04%. Firefox prints
a deprecation message for Attr.specified.

BUG=305105

Review URL: https://codereview.chromium.org/119173005

git-svn-id: svn://svn.chromium.org/blink/trunk@164578 bbb929c8-8fbe-4397-9dbb-9b2b20218538