Separate CSSValueID mappings generator from ComputedStyleBase generator.

We currently generate mappings between CSSValueID and platform enums
inside the ComputedStyleBase generator. However, because such mappings
do not exist for nonproperty fields, it no longer makes sense for us
to generate these mappings inside make_computed_style_base.py.

This patch moves the generator for CSSValueIDMappingsGenerated to
its own file, so that it is not affected by nonproperty fields.

BUG=628043

Review-Url: https://codereview.chromium.org/2794853002
Cr-Commit-Position: refs/heads/master@{#463955}
3 files changed