GN: Fix Mac bootstrap compile error

Mac C++ compiler is not able to convert a base::ListValue
unique pointer object to a base::Value unique pointer object.

R=dpranke@chromium.org

Change-Id: I07072578bd432c1a6ea13ca9757c75c3da37107c
Reviewed-on: https://chromium-review.googlesource.com/941208Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yngve Pettersen <yngve@vivaldi.com>
Cr-Commit-Position: refs/heads/master@{#540348}

tools/gn/visibility.cc

@@ -88,7 +88,7 @@ std::unique_ptr<base::Value> Visibility::AsValue() const {
   auto res = std::make_unique<base::ListValue>();
   for (const auto& pattern : patterns_)
     res->AppendString(pattern.Describe());
-  return res;
+  return std::move(res);
 }
 
 // static