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