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 70fe08503eafb50990502a894e6775ca2451800d..4b47a1c93296e1d825b92da1819136412cc13012 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.
      */