Move angle_enable_* inside declare_args(). 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>