This changes from returning a vector to directly returning a
pointer to the binary data for the serialized state.

The second cleanup is to use a ContextID as a wrapped type
which simplifies the output formatting code.

Bug: angleproject:5247
Change-Id: Ieb8afdb9326a12968dd2d69c05e1ed811b93abff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2506198Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>