[css-properties-values-api] Put image and transform values behind flags.

This CL adds new runtime flags which makes it possible to ship without
support for <image>, <transform-function> and <transform-list>.

Also replace comment near CSSPaintAPIArguments with 'depends_on'. (Added
before I knew 'depends_on' existed).

R=chrishtr@chromium.org

Bug: 641877
Change-Id: Ic50bdaffc84a7037c23841c9eb53eb9b331ad680
Reviewed-on: https://chromium-review.googlesource.com/c/1273061
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601534}
5 files changed