add peekPixels to canvas

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 016e2babc88d9048255a9f90174e4a159f0bee9c..a9468b16d18b902476258c8ae887278aefc467f6 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -369,7 +369,10 @@ protected:
 protected:
     // default impl returns NULL
     virtual SkSurface* newSurface(const SkImageInfo&);
-
+    
+    // default impl returns NULL
+    virtual const void* peekPixels(SkImageInfo*, size_t* rowBytes);
+    
     /**
      *  Leaky properties are those which the device should be applying but it isn't.
      *  These properties will be applied by the draw, when and as it can.