Replace source pointer in cc::CopyOutputRequest with a base::UnguessableToken

cc/surfaces/surface.cc

Index: cc/surfaces/surface.cc
diff --git a/cc/surfaces/surface.cc b/cc/surfaces/surface.cc
index 20d34011a58d5aa0bff6e4236856476a219fe560..f6c0645d3b996738452c36312ae195ba93b71519 100644
--- a/cc/surfaces/surface.cc
+++ b/cc/surfaces/surface.cc
@@ -86,13 +86,14 @@ void Surface::RequestCopyOfOutput(
     std::vector<std::unique_ptr<CopyOutputRequest>>& copy_requests =
         current_frame_->render_pass_list.back()->copy_requests;
 
-    if (void* source = copy_request->source()) {
+    if (copy_request->has_source()) {
+      auto source = copy_request->source();
       // Remove existing CopyOutputRequests made on the Surface by the same
       // source.
       auto to_remove =
           std::remove_if(copy_requests.begin(), copy_requests.end(),
                          [source](const std::unique_ptr<CopyOutputRequest>& x) {
-                           return x->source() == source;
+                           return x->has_source() && x->source() == source;
                          });
       copy_requests.erase(to_remove, copy_requests.end());
     }