Add support for creating a shared vulkan test context

Bug: skia:
Change-Id: I997c6269e4676bf4cedddcd87e71d107053678bb
Reviewed-on: https://skia-review.googlesource.com/16905
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed