Reland "Move creating encoder to VideoStreamEncoder."

This is a reland of fb82fcc7f9c414dc8ba1ddd314e9524fee54cb80

Original change's description:
> Move creating encoder to VideoStreamEncoder.
>
> This used to be in WebRtcVideoChannel::WebRtcVideoSendStream.
> One implication is that encoder is not created until the first
> frame arrives, and some of the tests needed updates to emit a
> frame or two.
>
> Bug: webrtc:8830
> Change-Id: I78169b2bb4dfa4197b4b4229af9fd69d0f747835
> Reviewed-on: https://webrtc-review.googlesource.com/64885
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22905}

TBR=magjed@webrtc.org,kwiberg@webrtc.org

Bug: webrtc:8830
Change-Id: I9565095ea1880fb49d15111198c08b2fcb84f18c
Reviewed-on: https://webrtc-review.googlesource.com/70740
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22930}

test/function_video_encoder_factory.h

@@ -0,0 +1,59 @@
+/*
+ *  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_ENCODER_FACTORY_H_
+#define TEST_FUNCTION_VIDEO_ENCODER_FACTORY_H_
+
+#include <functional>
+#include <memory>
+#include <utility>
+#include <vector>
+
+#include "api/video_codecs/video_encoder_factory.h"
+
+namespace webrtc {
+namespace test {
+
+// An encoder factory producing encoders by calling a supplied create
+// function.
+class FunctionVideoEncoderFactory final : public VideoEncoderFactory {
+ public:
+  explicit FunctionVideoEncoderFactory(
+      std::function<std::unique_ptr<VideoEncoder>()> create)
+      : create_(std::move(create)) {
+    codec_info_.is_hardware_accelerated = false;
+    codec_info_.has_internal_source = false;
+  }
+
+  // Unused by tests.
+  std::vector<SdpVideoFormat> GetSupportedFormats() const override {
+    RTC_NOTREACHED();
+    return {};
+  }
+
+  CodecInfo QueryVideoEncoder(
+      const SdpVideoFormat& /* format */) const override {
+    return codec_info_;
+  }
+
+  std::unique_ptr<VideoEncoder> CreateVideoEncoder(
+      const SdpVideoFormat& /* format */) override {
+    return create_();
+  }
+
+ private:
+  const std::function<std::unique_ptr<VideoEncoder>()> create_;
+  CodecInfo codec_info_;
+};
+
+}  // namespace test
+}  // namespace webrtc
+
+#endif  // TEST_FUNCTION_VIDEO_ENCODER_FACTORY_H_