Speculative fix for one-time ASAN crasher.

The stack traces seem to indicate that the scrollbar was being destroyed
when the HitTestResult goes out of scope but we were keeping a raw pointer
to it and accessing it after the HitTestResult is destroyed.

BUG=358813

Review URL: https://codereview.chromium.org/237883003

git-svn-id: svn://svn.chromium.org/blink/trunk@172372 bbb929c8-8fbe-4397-9dbb-9b2b20218538