Add a RELEASE_ASSERT() to invesigate some mysterious crashes

This change adds a RELEASE_ASSERT() which should replaces
a series of crashes we're seeing in the crash report.

BUG=352043
R=kouhei,japhet@chromium.org,eseidel

Review URL: https://codereview.chromium.org/195893025

git-svn-id: svn://svn.chromium.org/blink/trunk@169297 bbb929c8-8fbe-4397-9dbb-9b2b20218538