SkPDF: Use DeviceGray for alpha masks.

See https://groups.google.com/forum/#!topic/pdf-dev/qmdjg3w7D8E

BUG=chromium:742058
Change-Id: Ia8c204a92cb2b9fc33fab60826a95e359db65f08
Reviewed-on: https://skia-review.googlesource.com/26822
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
2 files changed