The constructor of gl::Context creates a bunch of zero and helper objects
but it is not current at this time.  If the backend if not using virtualized
contexts this causes the created objects to be created on the wrong native
context.

BUG=angleproject:2464

Change-Id: I9718df87d0afeb08729920363d362d5f891061ed
Reviewed-on: https://chromium-review.googlesource.com/1048114Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>