Store context type (rather than backend type) in ContextInfo

Change-Id: I7dbfb735f1f094fa0a025b84f69e06a3533f2815
Reviewed-on: https://skia-review.googlesource.com/19101
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed