[CSS Typed OM] Re-implement ToCSSValue and AsMatrix for CSSRotation

I previously commented these out when I deleted obsolete number and
length types, but now I can put it back.

Added tests, and also removed a test that didn't seem useful.

BUG=545318

Review-Url: https://codereview.chromium.org/2957603002
Cr-Commit-Position: refs/heads/master@{#484232}
7 files changed