Added api_methods flag to CSSProperties.json5.

A part of Project Ribbon, separating the parsing logic for CSS
properties from the parser into an API.

This patch adds an api_methods=["method1", "method2"] flag to
CSSProperties.json5, which specifies which methods have been
implemented in the API implementation of that property. This is so
that when API .h files are generated, only methods implemented for
that property are defined.

It also adds "parseSingleValue" to the list of api_methods for all
existing properties with an API that implements parseSingleValue.

Before, when a method was added to the API, this method had to be
implemented in all API files - however, not every API should hold
each method. In this new design, to add a method to the API:
- Add the method name to the valid_values array in CSSProperties.json5.
- Add the method to CSSPropertyAPIFiles.h.tmpl
- Add the method name to the api_methods list in CSSProperties.json5
  for each property where it is implemented.

BUG=668012

Review-Url: https://codereview.chromium.org/2654403003
Cr-Commit-Position: refs/heads/master@{#447427}
4 files changed