Implements CSSPropertyAPI for the font-size-adjust property.

A part of Project Ribbon, separating the parsing logic for CSS
properties from the parser into an API. This patch removes
CSSPropertyFontSizeAdjust 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 CSSPropertyAPIFontSizeAdjust.cpp to the BUILD.gn file.
- Adds api_class flag to CSSProperties.in, which indicates that
  CSSPropertyAPIFontSizeAdjust.h is generated.
- Moves the parsing logic for page from CSSPropertyParser.cpp to
  CSSPropertyAPIFontSizeAdjust.cpp, which implements
  CSSPropertyAPI.h.

BUG=668012

Review-Url: https://codereview.chromium.org/2617443005
Cr-Commit-Position: refs/heads/master@{#442814}