Add method to sk_gpu_test::TestContext to automatically restore the previous context.

The motivation for this is to allow a GM to create a GL context, do some some work in it, and then return to the context that was set when it was invoked.

Change-Id: Ie8496072a10f8f3ff36a08889e593a6ca961b61a
Reviewed-on: https://skia-review.googlesource.com/70720
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
21 files changed