Remove a triggering ASSERT

https://codereview.chromium.org/25492005 made a mistake
when converting an ASSERT so it ended up triggering.
The new ASSERT is not right so let's remove it.

TBR=eseidel@chromium.org
BUG=260898

Review URL: https://codereview.chromium.org/25775002

git-svn-id: svn://svn.chromium.org/blink/trunk@158743 bbb929c8-8fbe-4397-9dbb-9b2b20218538