Do not reset mouse focus flag on keydown at anchor elements

BUG=412251
R=pdr@chromium.org
TEST=./Tools/Scripts/run-webkit-tests fast/events/click-*

Review URL: https://codereview.chromium.org/562653002

git-svn-id: svn://svn.chromium.org/blink/trunk@181844 bbb929c8-8fbe-4397-9dbb-9b2b20218538