[css-typed-om] Rejig CSSResourceValue hierarchy.

We implement the new CSSImageValue, which is an opaque object with no
constructor. We also had to change the testsuite.js a bit to test it.

Bug: 545318
Change-Id: I0ae719c5380e01ff9423e0ff0684afe547de1bb0
Reviewed-on: https://chromium-review.googlesource.com/934001
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: nainar <nainar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538730}
28 files changed