Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 37cbff50e2789dae0f36ca2cdd7df7a585213d35..e545baea93f54f410ad5d98c8db77ab9e3633dde 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -25,7 +25,7 @@
     // TODO: can we stop supporting kUnknown in SkBitmkapDevice?
     if (kUnknown_SkColorType == info.colorType()) {
         if (newAlphaType) {
-            *newAlphaType = kUnknown_SkAlphaType;
+            *newAlphaType = kIgnore_SkAlphaType;
         }
         return true;
     }