make info real in SkPixelRef, and add bitmap::asImageInfo

include/core/SkBitmap.h

Index: include/core/SkBitmap.h
diff --git a/include/core/SkBitmap.h b/include/core/SkBitmap.h
index cd85b6a9b1ad019102ba93271a6bec69ff4d97a1..c0e299ab94ba8bc3c3caeefb3796b4d24e12de5a 100644
--- a/include/core/SkBitmap.h
+++ b/include/core/SkBitmap.h
@@ -250,6 +250,13 @@ public:
 
     bool setConfig(const SkImageInfo& info, size_t rowBytes = 0);
 
+    /**
+     *  If the bitmap's config can be represented as SkImageInfo, return true,
+     *  and if info is not-null, set it to the bitmap's info. If it cannot be
+     *  represented as SkImageInfo, return false and ignore the info parameter.
+     */
+    bool asImageInfo(SkImageInfo* info) const;
+
     /** Use this to assign a new pixel address for an existing bitmap. This
         will automatically release any pixelref previously installed. Only call
         this if you are handling ownership/lifetime of the pixel memory.