Handle GrContext creation failure in DDL

TBR=bsalomon@google.com
Change-Id: Ib34448a60cb127ef89bcb0b1f85d07abc91af582
Reviewed-on: https://skia-review.googlesource.com/114569
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed