mark bitmap immutable in bench

So we don't incidentally measure the cost of the copy

Change-Id: I6f7e2119792423b0857764b3b557a9f838bb43fb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334423
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed