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>
diff --git a/src/pdf/SkPDFDevice.h b/src/pdf/SkPDFDevice.h
index 51a9b10..b9ee881 100644
--- a/src/pdf/SkPDFDevice.h
+++ b/src/pdf/SkPDFDevice.h
@@ -212,7 +212,8 @@
 
     void init();
     void cleanUp();
-    sk_sp<SkPDFObject> makeFormXObjectFromDevice();
+    // Set alpha to true if making a transparency group form x-objects.
+    sk_sp<SkPDFObject> makeFormXObjectFromDevice(bool alpha = false);
 
     void drawFormXObjectWithMask(int xObjectIndex,
                                  sk_sp<SkPDFObject> mask,