Fixes to shared context test API

Fixes a bug in Windows shared context creation, and makes the API
less fiddly.

BUG=skia:

Change-Id: Ia32b2e3b4816e0b8d7e9be92c22a182ca1393177
Reviewed-on: https://skia-review.googlesource.com/8965
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed