use colortype instead of config

patch from issue 172063004

BUG=skia:
R=scroggo@google.com

Review URL: https://codereview.chromium.org/173893002

git-svn-id: http://skia.googlecode.com/svn/trunk@13520 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index 2b449dc..1c73d95 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -165,6 +165,13 @@
         return info;
     }
 
+    static SkImageInfo MakeUnknown(int width, int height) {
+        SkImageInfo info = {
+            width, height, kUnknown_SkColorType, kIgnore_SkAlphaType
+        };
+        return info;
+    }
+
     int width() const { return fWidth; }
     int height() const { return fHeight; }
     SkColorType colorType() const { return fColorType; }