Implements CSSPropertyAPI for the flex-basis property.

A part of Project Ribbon, separating the parsing logic for CSS
properties from the parser into an API. This patch removes
CSSPropertyFlexBasis from the switch statement in parseSingleValue,
and calls the API instead.

A function pointer to the parseSingleValue function from the API for the
page property is stored in a CSSPropertyDescriptor, and is called from
CSSPropertyParser.

This patch:
- Adds CSSPropertyAPIFlexBasis.cpp to the BUILD.gn file.
- Adds api_class flag to CSSProperties.in, which indicates that
  CSSPropertyAPIFlexBasis.h is generated.
- Moves the parsing logic for page from CSSPropertyParser.cpp to
  CSSPropertyAPIFlexBasis.cpp, which implements CSSPropertyAPI.h.

BUG=668012

Review-Url: https://codereview.chromium.org/2613843002
Cr-Commit-Position: refs/heads/master@{#442815}