slice GMs by native font platform, not by emoji format

Right now the native font platform implies what emoji formats are
supported, and we hope to support more than one per platform.  Let's
get these formats out of the name.

As with most other font-y things, only the NativeFont bots are going to
get back anything interesting now.  The others will see no emoji font
and an empty emoji sample text string.

I'm going to look at a pre-baked testing SkTypeface that serves as an
emoji font for the non-NativeFont bots next.

Change-Id: Ie1374fc0e988bfe20ae21208e2f7e0a66a68fcb1
Reviewed-on: https://skia-review.googlesource.com/71762
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed