Add "Narrow" RGB gamut to DM

This is an artificially small color space, for testing rendering with
values that fall outside of [0,1]. All of the saturated sRGB colors used
in our tests will end up landing outside of [0,1] in this space, so we can
verify that nothing that crashes, or produces wildly incorrect results.

Of course, many GMs *do* produce wildly incorrect results, but I need
to catalog all of those and start figuring out why.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3532

Change-Id: Idb44ee6250f2d7198e634755cd2b1f3bc8f15412
Reviewed-on: https://skia-review.googlesource.com/3532
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed