Add keyword_subset field template and generate text-decoration-line.

This patch adds support for generating 'keyword_subset' fields in
ComputedStyleBase. 'keyword_subset' fields can take on a subset of
keywords. For example, the text-decoration-line property can be any
combination of underline, overline, line-through or blink.

To support 'keyword_subset', we create a new class in the generator
called Enum to represent both 'keyword' and 'keyword_subset' enums.
In ComputedStyleBaseConstants.h.tmpl, we generate different code
depending on whether the enum is a set or not. Unlike 'keyword' enums,
a 'keyword_subset' enum has explicit values and supports bitwise operators.

Diff of generated files:
https://gist.github.com/darrnshn/70d7a9e1ef02aee205b5c1d42d4df3bb/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2914613002
Cr-Commit-Position: refs/heads/master@{#484037}
9 files changed