When a surface is backed by an external render target force a copy on image snap

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1694943002

Review URL: https://codereview.chromium.org/1694943002
diff --git a/tests/SurfaceTest.cpp b/tests/SurfaceTest.cpp
index 7449f21..8166bba 100644
--- a/tests/SurfaceTest.cpp
+++ b/tests/SurfaceTest.cpp
@@ -439,9 +439,7 @@
         {
             SkAutoTUnref<SkSurface> surface(
                 SkSurface::NewRenderTargetDirect(texture->asRenderTarget()));
-            // We should be able to pass true here, but disallowing copy on write for direct GPU
-            // surfaces is not yet implemented.
-            test_unique_image_snap(reporter, surface, false, imageBackingStore,
+            test_unique_image_snap(reporter, surface, true, imageBackingStore,
                                    surfaceBackingStore);
         }
         texture->unref();