This allows enabling and disabling parts of the build via gn args,
similar to what gyp allowed.

BUG=angleproject:1569

Change-Id: Ia95c134f98f8941408df96b1d4e2563e713a9ef2
Reviewed-on: https://chromium-review.googlesource.com/838260
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>