Revert "Don't change mask format in generateImage."

This reverts commit 1d12fa47f6a05c776255b47356f88f0b21b3c4a7.

Reason for revert: hit assert

Original change's description:
> Don't change mask format in generateImage.
> 
> Change-Id: I879e5ace9bff4ea6edc5e522d3c48a4edd0ea242
> Reviewed-on: https://skia-review.googlesource.com/c/174064
> Reviewed-by: Herb Derby <herb@google.com>
> Commit-Queue: Ben Wagner <bungeman@google.com>

TBR=bungeman@google.com,herb@google.com

Change-Id: I6000da4ee8f4f11ec270c180820fa4a7793a761e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/174301
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed