CSS Animations: Add interpolation tests for webkit-transform

There are currently no test failures for web-animations-css. There are some expected failures with the legacy (non web-animations-css) implementation due to NOOP transforms being reported as "matrix(1, 0, 0, 1, 0, 0)" or similar instead of "none".

BUG=257591

Review URL: https://codereview.chromium.org/27455002

git-svn-id: svn://svn.chromium.org/blink/trunk@160389 bbb929c8-8fbe-4397-9dbb-9b2b20218538