Convert video quality test from a TEST_F to a TEST fixture.

The purpose is to make the fixture reusable in downstream
projects. The CL adds the following things to API:

- api/test/video_quality_test_fixture.h
- api/test/create_video_quality_test_fixture.h

The following things are moved to API:

- call/bitrate_constraints.h (api/bitrate_constraints.h)
- call/simulated_network.h (api/test/simulated_network.h)
- call/media_type.h (api/mediatypes.h)

These are required by the params struct passed to the
fixture. I didn't attempt to split the params struct into
an internal-only and public version in this CL, and as
a result we need to pull in the above things. They are
quite harmless though, so I think it's worth it in order
to avoid splitting up the test config struct.

This CL doesn't solve all the problems we need to
implement downstream tests; we probably need to upstream
tracing variants of FakeNetworkPipe for instance, but
that will come later. This puts in place the basic
structure for now.

Bug: None
Change-Id: I35e26ed126fad27bc7b2a465400291084f6ac911
Reviewed-on: https://webrtc-review.googlesource.com/69601
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23714}

api/bitrate_constraints.h

@@ -0,0 +1,41 @@
+/*
+ *  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 API_BITRATE_CONSTRAINTS_H_
+#define API_BITRATE_CONSTRAINTS_H_
+
+#include <algorithm>
+
+namespace webrtc {
+// TODO(srte): BitrateConstraints and BitrateSettings should be merged.
+// Both represent the same kind data, but are using different default
+// initializer and representation of unset values.
+struct BitrateConstraints {
+  int min_bitrate_bps = 0;
+  int start_bitrate_bps = kDefaultStartBitrateBps;
+  int max_bitrate_bps = -1;
+
+ private:
+  static constexpr int kDefaultStartBitrateBps = 300000;
+};
+
+// Like std::min, but considers non-positive values to be unset.
+template <typename T>
+static T MinPositive(T a, T b) {
+  if (a <= 0) {
+    return b;
+  }
+  if (b <= 0) {
+    return a;
+  }
+  return std::min(a, b);
+}
+}  // namespace webrtc
+#endif  // API_BITRATE_CONSTRAINTS_H_