Implement support for the 'transform-box' property

Spec:

 https://drafts.csswg.org/css-transforms-1/#transform-box

Intent to implement:

 https://groups.google.com/a/chromium.org/d/topic/blink-dev/v8g3QF8pRF8

Implemented behind the CSSTransformBox Blink runtime flag.
Add some basic tests and a smoke test for the "legacy" transform-box
behavior. Existing tests are adjusted to be compatible with the new
behavior.
The test svg/transforms/transform-origin-css-property.xhtml is "fixed"
by adding units when required (unitless numbers are not valid in inline
style.)

BUG=595829

Review-Url: https://codereview.chromium.org/2786643003
Cr-Commit-Position: refs/heads/master@{#462266}
24 files changed