Implement parseShorthand API for shorthand properties, "overflow", "font" and "font-variant"

This cl contains the following changes
* API impl of the three shorthand properties (CSSShorthandPropertyAPIOverflow,
 CSSShorthandPropertyAPIFont, CSSShorthandPropertyAPIFontVariant).

* Moved ConsumeFontVariantCSS21 from CSSPropertyParser.cpp to CSSPropertyFontUtils.
- this function is being used by CSSShorthandPropertyAPIFont and also another function
from CSSPropertyParser.cpp.

* Changed CSSPropertyParser.cpp to use the new API parseShorthand for the three properties
above. Additional changes are:
- moved two helper functions (ConsumeSystemFont and ConsumeFont) to be private functions of CSSShorthandPropertyAPIFont.
- removed ConsumeFontVariantShorthand that's no longer needed.

* Added the properties API to BUILD file and CSSProperties.json5.

* Changed AddProperty signature so that "implicit" is now an enum.
- We previously passed on bool literals to this function. This change makes the function follow the style guide.

Diff: https://gist.github.com/810c7e46324ff8db8efb6f34ddfdad42/revisions
Review-Url: https://codereview.chromium.org/2938983002
Cr-Commit-Position: refs/heads/master@{#481037}