Fix crash in 2D canvas drawSystemFocusRing with invalid argument.

Added missing null ptr check.

BUG=353248

Review URL: https://codereview.chromium.org/203293005

git-svn-id: svn://svn.chromium.org/blink/trunk@169468 bbb929c8-8fbe-4397-9dbb-9b2b20218538