Move comma operator parsing to ParseContext This change is pure refactoring. It will help with fixing comma operator return value qualifier bug. BUG=angleproject:911 BUG=angleproject:1201 Change-Id: I481c337adbaf789cc959c8a1106f99ad7275e1e5 Reviewed-on: https://chromium-review.googlesource.com/311170 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com>