Don't assert trying to make an MSAA surface with Metal, just fail

Change-Id: I66b18d753f0e78cc1d647328322d12c037fa9ed5
Reviewed-on: https://skia-review.googlesource.com/149484
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed