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

src/gpu/GrGpuResourceCacheAccess.h

Index: src/gpu/GrGpuResourceCacheAccess.h
diff --git a/src/gpu/GrGpuResourceCacheAccess.h b/src/gpu/GrGpuResourceCacheAccess.h
index 8c272420a7180dfe0c0f8f65d2086d89a6cefdb5..a0025582f5d34ee335fde0a0e0764db374b39963 100644
--- a/src/gpu/GrGpuResourceCacheAccess.h
+++ b/src/gpu/GrGpuResourceCacheAccess.h
@@ -31,11 +31,6 @@ private:
     }
 
     /**
-     * Is the resource object wrapping an externally allocated GPU resource?
-     */
-    bool isExternal() const { return fResource->isExternal(); }
-
-    /**
      * Is the resource object wrapping an externally allocated GPU resource that Skia has not taken
      * ownership of.
      */