Add UseCounters for the Attr.value getter and setter

This is to determine if it could be made readonly. The getter is
measured to learn the relative usage, and is most likely above any
threshold for removal.

BUG=none

Review URL: https://codereview.chromium.org/203193010

git-svn-id: svn://svn.chromium.org/blink/trunk@169541 bbb929c8-8fbe-4397-9dbb-9b2b20218538