[css-ui] Add support for caret-color property

The test basically adds the new CSS property "caret-color":

"caret-color" is a regular color property but it accepts also "auto".
To support this a new class StyleAutoColor has been created,
including a new flag to know if the color is auto, currentColor
or a specified one.

Finally the patches modifies CaretBase::paintCaret() to use
that property instead of "color" as it's been doing until now.

We're using the test suite from the W3C CSS WG,
which was imported on r434986.
This CL manually imports some pixel tests too.

This is based on a patch by Claudio Saavedra <csaavedra@igalia.com>.


Review-Url: https://codereview.chromium.org/2520873002
Cr-Commit-Position: refs/heads/master@{#435338}
106 files changed