add testing flag to force rasterpipeline

Bug: skia:
Change-Id: I8e195e90dc00b9be9072858086f7541e993428da
Reviewed-on: https://skia-review.googlesource.com/18585
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed