Add ability to extract YUV planes from SkImage

include/core/SkImageInfo.h

Index: include/core/SkImageInfo.h
diff --git a/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index 4b82ae6e2fb317cf2414cb09801d72efc26de423..58635db5e13f5083ee7433fed1c864c890234ede 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -206,6 +206,11 @@ public:
                            kLinear_SkColorProfileType);
     }
 
+    static SkImageInfo MakeGray8(int width, int height) {
+        return SkImageInfo(width, height, kGray_8_SkColorType, kPremul_SkAlphaType,
+                           kLinear_SkColorProfileType);
+    }
+
     static SkImageInfo MakeUnknown(int width, int height) {
         return SkImageInfo(width, height, kUnknown_SkColorType, kUnknown_SkAlphaType,
                            kLinear_SkColorProfileType);