add SkCanvas constructor that explicitly creates no_config with dimensions

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 5a5cc65f0b3e69621582b0f72b45bc4c8c5dda8a..c98ad996f0786caccf75d3f74dca3dbc68edda10 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -49,8 +49,19 @@ class SK_API SkCanvas : public SkRefCnt {
 public:
     SK_DECLARE_INST_COUNT(SkCanvas)
 
+    /**
+     *  Creates an empty canvas with no backing device/pixels, and zero
+     *  dimensions.
+     */
     SkCanvas();
 
+    /**
+     *  Creates a canvas of the specified dimensions, but explicitly not backed
+     *  by any device/pixels. Typically this use used by subclasses who handle
+     *  the draw calls in some other way.
+     */
+    SkCanvas(int width, int height);
+
     /** Construct a canvas with the specified device to draw into.
 
         @param device   Specifies a device for the canvas to draw into.