These were both id values for the context. This CL consolidates to the
monotonically increasing ID since it is both safer and easier to debug.

Makes life easier when indexing feedback loops by context ID.

Bug: angleproject:4517
Change-Id: I28b40fed4e6e68ea72742f7defde5f8638f4cd47
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2134408Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>