Allows benchmarks to override GrContextOptions.
Removes the ability to use the same GrContext for all benchmarks in a config.
Change-Id: I5ab9f6e81055451ac912a66537843d1a49f3b479
Reviewed-on: https://skia-review.googlesource.com/34080
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
