Make all SkSurface_* -related devices all have a device

include/core/SkDevice.h

Index: include/core/SkDevice.h
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 9f3ac580080590c36989f4a657040e6a79092317..33c8dfa6270e4ee136bf0df1518e3d4b64fa04d2 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -123,6 +123,11 @@ public:
 #endif
     };
 
+    /** This makes the contents of this device undefined.
+     * See SkCanvas::discard()
+     */
+    virtual void discard() { }
+
 protected:
     enum Usage {
        kGeneral_Usage,
@@ -367,6 +372,8 @@ private:
     friend class SkNoPixelsBitmapDevice;
 
     friend class SkSurface_Raster;
+    friend class SkSurface_Gpu;
+    friend class SkGpuDevice;
 
     // used to change the backend's pixels (and possibly config/rowbytes)
     // but cannot change the width/height, so there should be no change to