Add instanced rendering command line flag to viewer.

Previously this could only be turned on at runtime.

Change-Id: I1b626584fba17fcf8ff64135dd93f98c7f40821d
Reviewed-on: https://skia-review.googlesource.com/9445
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed