Refactor SimulcastTestUtility into SimulcastTestFixture{,Impl}

This will allow exposing the interface to downstream users that
want to test VP8 simulcast. No functional changes to the tests
themselves are expected.

Bug: webrtc:9281
Change-Id: I4128b8f35a4412c5b330cf55c8dc0e173d4570da
Reviewed-on: https://webrtc-review.googlesource.com/77361
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23469}

test/function_video_decoder_factory.h

@@ -0,0 +1,51 @@
+/*
+ *  Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef TEST_FUNCTION_VIDEO_DECODER_FACTORY_H_
+#define TEST_FUNCTION_VIDEO_DECODER_FACTORY_H_
+
+#include <functional>
+#include <memory>
+#include <utility>
+#include <vector>
+
+#include "api/video_codecs/sdp_video_format.h"
+#include "api/video_codecs/video_decoder_factory.h"
+#include "rtc_base/checks.h"
+
+namespace webrtc {
+namespace test {
+
+// A decoder factory producing decoders by calling a supplied create function.
+class FunctionVideoDecoderFactory final : public VideoDecoderFactory {
+ public:
+  explicit FunctionVideoDecoderFactory(
+      std::function<std::unique_ptr<VideoDecoder>()> create)
+      : create_(std::move(create)) {}
+
+  // Unused by tests.
+  std::vector<SdpVideoFormat> GetSupportedFormats() const override {
+    RTC_NOTREACHED();
+    return {};
+  }
+
+  std::unique_ptr<VideoDecoder> CreateVideoDecoder(
+      const SdpVideoFormat& /* format */) override {
+    return create_();
+  }
+
+ private:
+  const std::function<std::unique_ptr<VideoDecoder>()> create_;
+};
+
+}  // namespace test
+}  // namespace webrtc
+
+#endif  // TEST_FUNCTION_VIDEO_DECODER_FACTORY_H_