Partial revert of r170233

This CL triggered a crash on this line of code. The comment at the top of the
function gives some hints as to why, but I wasn't able to construct a test
case that actually hits this code.

BUG=359547

Review URL: https://codereview.chromium.org/239393005

git-svn-id: svn://svn.chromium.org/blink/trunk@171907 bbb929c8-8fbe-4397-9dbb-9b2b20218538