cc: Keep PaintImage in DrawImage.

cc/paint/paint_image.h

Index: cc/paint/paint_image.h
diff --git a/cc/paint/paint_image.h b/cc/paint/paint_image.h
index 1dc39ae5551da64cb75a7acb57335a921bc211e2..53f73576352715bf8af2c51b2bf1e3cd37efb7fe 100644
--- a/cc/paint/paint_image.h
+++ b/cc/paint/paint_image.h
@@ -42,7 +42,7 @@ class CC_PAINT_EXPORT PaintImage {
   PaintImage& operator=(const PaintImage& other);
   PaintImage& operator=(PaintImage&& other);
 
-  bool operator==(const PaintImage& other);
+  bool operator==(const PaintImage& other) const;
   explicit operator bool() const { return sk_image_; }
 
   Id stable_id() const { return id_; }