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

This reverts commit 1e09e461d2ffcf8b07242cfe93dd7d12c4d75866.

Change-Id: I95d5544a7baaa078536790493ce4119816a77e94
Reviewed-on: https://skia-review.googlesource.com/72903
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
21 files changed