Add UseCounter for case-insensitive attribute value selector matching

More specifically, count the cases where the result would be different
if a currently (automatically) case-insensitive attribute value match
was made case-sensitive instead, as is the current status of the spec.

BUG=327060

Review URL: https://codereview.chromium.org/140323003

git-svn-id: svn://svn.chromium.org/blink/trunk@165740 bbb929c8-8fbe-4397-9dbb-9b2b20218538