Rename FetchGSUtil GetCachedGSContext, no longer a contextmanager.

The only use case we have for the gsutil binary returned by FetchGSUtil
is to create a GSContext, so fold in functionality to create a
GSContext, and rename the function appropriately.

We also remove the read lock we have on the cached gsutil, since the
common cache is not intended to have LRU cleanup.  This allows
ChromeSDK to instantiate its own GSContext() object, which simplifies
its interface.

BUG=None
TEST=Unit tests.

Change-Id: Ie03272eafd0520c2774b55c27dd8cb7ec1262bb0
Reviewed-on: https://gerrit.chromium.org/gerrit/42968
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
6 files changed