remove guard for SkImageGenerator api

BUG=skia:

Change-Id: I0c01fbf935be5d933aa2ef67a0fb2edba4941fa7
Reviewed-on: https://skia-review.googlesource.com/8835
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed