Make WebkitCSSMatrix an alias of DOMMatrix

DOMMatrix was changed sufficiently that WebKitCSSMatrix can be made a simple alias of it.

Intent-to-ship Link :
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/
cAiTOdSGyes/Su7dlx32EQAJ

BUG=581955

Review-Url: https://codereview.chromium.org/2709763004
Cr-Commit-Position: refs/heads/master@{#479481}
23 files changed