Allow client to force an SkImage snapshot to be unique (and uniquely own its backing store).

include/core/SkSurface.h

Index: include/core/SkSurface.h
diff --git a/include/core/SkSurface.h b/include/core/SkSurface.h
index 45262d78e63d3d30351c43ccd22ae3c2442ad0a3..d5a66b4bb7f8fb0a5417fada09ff08aad8bd6e5a 100644
--- a/include/core/SkSurface.h
+++ b/include/core/SkSurface.h
@@ -237,6 +237,18 @@ public:
     SkImage* newImageSnapshot(Budgeted = kYes_Budgeted);
 
     /**
+     * In rare instances a client may want a unique copy of the SkSurface's contents in an image
+     * snapshot. This enum can be used to enforce that the image snapshot's backing store is not
+     * shared with another image snapshot or the surface's backing store. This is generally more
+     * expensive. This was added for Chromium bug 585250.
+     */
+    enum ForceUnique {
+        kNo_ForceUnique,
+        kYes_ForceUnique
+    };
+    SkImage* newImageSnapshot(Budgeted, ForceUnique);
+
+    /**
      *  Though the caller could get a snapshot image explicitly, and draw that,
      *  it seems that directly drawing a surface into another canvas might be
      *  a common pattern, and that we could possibly be more efficient, since