Fixup enum name in SkFlattenable.

`SkShaderBase` is an implementation-detail of SkShader and doesn't need
to be in this enum name.

Change-Id: Ieb40a09ae489d4a1e11dfd306d20a8b94ee05ef9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435017
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed