Properly test tap hit testing by supplying a radius

Depends on chromium change https://codereview.chromium.org/358963007/

BUG=381728

Review URL: https://codereview.chromium.org/356843009

git-svn-id: svn://svn.chromium.org/blink/trunk@178105 bbb929c8-8fbe-4397-9dbb-9b2b20218538