Adding missing null ptr check in MouseEventHitRegion::region

BUG=387561
TEST=clusterfuzz key=5611287658627072

Review URL: https://codereview.chromium.org/350643004

git-svn-id: svn://svn.chromium.org/blink/trunk@176768 bbb929c8-8fbe-4397-9dbb-9b2b20218538