Simplify initialization of test FrameGeneratorCapturerConfig.

Allowing assignment of the AutoOpt fields:
AutoOpt<T> field = T();

Bug: webrtc:9883
Change-Id: I3fd73d29b4d8c6c6b72ae9ed5fb9511ae98af95e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169558
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30674}

test/frame_generator_capturer.h

@@ -29,6 +29,7 @@ namespace frame_gen_cap_impl {
 template <typename T>
 class AutoOpt : public absl::optional<T> {
  public:
+  using absl::optional<T>::optional;
   T* operator->() {
     if (!absl::optional<T>::has_value())
       this->emplace(T());