Add a Canvas::SaveAndUnscale() method for pixel-accurate drawing.

ui/gfx/canvas.h

Index: ui/gfx/canvas.h
diff --git a/ui/gfx/canvas.h b/ui/gfx/canvas.h
index 84a685cfb517f146fae5e67eae4bd50f30286b95..0038d9a307c2948b0b994f69276a6516ba92654a 100644
--- a/ui/gfx/canvas.h
+++ b/ui/gfx/canvas.h
@@ -169,6 +169,11 @@ class GFX_EXPORT Canvas {
   // until a balanced call to Restore() is made.
   void Save();
 
+  // Saves the drawing state, unscales by the image scale factor, and returns
+  // that factor.  This is useful when callers want to draw directly in the
+  // native scale.
+  float SaveAndUnscale();
+
   // As with Save(), except draws to a layer that is blended with the canvas
   // at the specified alpha once Restore() is called.
   // |layer_bounds| are the bounds of the layer relative to the current