GitLab

Add support for a new EnableIf attribute to conditionally
enable fields/types in mojom definitions.

In order to do this, mojom parsing and code generation has
been split into two separate GN stages. The mojom bindings
generator has been refactored to separate the logic for
parsing and code generation and the GN mojom template has
been updated to express these two distinct stages.

The parse stage now prunes a mojom's AST - filtering
definitions based on the enabled features. These
intermediate ASTs are then pickled to gen/ to be later
read by the code generation stage.

Bug: 676224
Change-Id: I5fc106b43e8ac48339c63c48f7ce42ba5145d174
Reviewed-on: https://chromium-review.googlesource.com/846399
Commit-Queue: Eve Martin-Jones <evem@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Chris Watkins <watk@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533796}