Count internal use of DOMImplementation::hasFeature() returning false

This is the only internal usage of DOMImplementation::hasFeature(), and
the only way that svg:requiredFeatures is observable, other than the
it->isEmpty() case.

If usage is low, DOMImplementation.hasFeature() and this internal usage
can be changed together, see
https://groups.google.com/a/chromium.org/d/msg/blink-dev/r2aARnIHG4k/_dN33g65mYgJ

Review URL: https://codereview.chromium.org/712813002

git-svn-id: svn://svn.chromium.org/blink/trunk@185133 bbb929c8-8fbe-4397-9dbb-9b2b20218538