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

src/image/SkSurface.cpp

Index: src/image/SkSurface.cpp
diff --git a/src/image/SkSurface.cpp b/src/image/SkSurface.cpp
index fed13a284beaed2202f5d2bde92f71032dcc777f..ea0cd682ac27445380e970d0f0117165dcf63d1c 100644
--- a/src/image/SkSurface.cpp
+++ b/src/image/SkSurface.cpp
@@ -163,10 +163,9 @@ SkCanvas* SkSurface::getCanvas() {
     return asSB(this)->getCachedCanvas();
 }
 
-SkImage* SkSurface::newImageSnapshot(Budgeted budgeted) {
-    SkImage* image = asSB(this)->getCachedImage(budgeted);
-    SkSafeRef(image);   // the caller will call unref() to balance this
-    return image;
+SkImage* SkSurface::newImageSnapshot(Budgeted budgeted, ForceUnique unique) {
+    // the caller will call unref() to balance this
+    return asSB(this)->refCachedImage(budgeted, unique);
 }
 
 SkSurface* SkSurface::newSurface(const SkImageInfo& info) {