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
Web Animations: Assert that convertToLength succeeds instead of returning Length(Undefined)

CSSPrimitiveValue::convertToLength should always match one of the cases
for conversion, so instead of returning Length(Undefined) if we somehow
fail and asserting in the callers we aren't undefined, simply assert we
don't fail.

BUG=322365

Review URL: https://codereview.chromium.org/108653006

git-svn-id: svn://svn.chromium.org/blink/trunk@164041 bbb929c8-8fbe-4397-9dbb-9b2b20218538