Handle weird mozilla extensions to reftest.list w/o crashing.

It looks like the w3c has some 'reftest.list' files that don't follow
the standard three-field format. Prior to this patch, we would
crash when we tried to read them.

R=ojan@chromium.org, abarth@chromium.org
BUG=329111

Review URL: https://codereview.chromium.org/108703005

git-svn-id: svn://svn.chromium.org/blink/trunk@164039 bbb929c8-8fbe-4397-9dbb-9b2b20218538