Fix the usage counting of CaptureAttributeAsEnum.

We were counting usage of this deprecated feature even when it was not used
in a deprecated form (ie. when the "capture" attribute of the <input>
element was not used as an enum). We now measure usage of both
CaptureAttributeAsEnum and the newly-added CaptureAttributeAsBoolean, so
that we know when usage of the deprecated form has dropped low enough
compared to the recommended one.

Additionally, correctly detect whether the capture attribute is being used
in a deprecated way: using !fastGetAttribute().isNull() at that point would
always return true -- we want isEmpty() instead.

R=jochen@chromium.org,tkent@chromium.org

Review URL: https://codereview.chromium.org/137233012

git-svn-id: svn://svn.chromium.org/blink/trunk@165868 bbb929c8-8fbe-4397-9dbb-9b2b20218538