Remove voe::TransmitMixer

TransmitMixer's functionality is moved into the AudioTransportProxy
owned by AudioState. This removes the need for an AudioTransport
implementation in VoEBaseImpl, which means that the proxy is no longer
a proxy, hence AudioTransportProxy is renamed to AudioTransportImpl.

In the short term, AudioState needs to know which AudioDeviceModule is
used, so it is added in AudioState::Config. AudioTransportImpl needs
to know which AudioSendStream:s are currently enabled to send, so
AudioState maintains a map of them, which is reduced into a simple
vector for AudioTransportImpl.

To encode and transmit audio,
AudioSendStream::OnAudioData(std::unique_ptr<AudioFrame> audio_frame)
is introduced, which is used in both the Chromium and standalone use
cases. This removes the need for two different instances of
voe::Channel::ProcessAndEncodeAudio(), so there is now only one,
taking an AudioFrame as argument. Callers need to allocate their own
AudioFrame:s, which is wasteful but not a regression since this was
already happening in the voe::Channel functions.

Most of the logic changed resides in
AudioTransportImpl::RecordedDataIsAvailable(), where two strange
things were found:

  1. The clock drift parameter was ineffective since
     apm->echo_cancellation()->enable_drift_compensation(false) is
     called during initialization.

  2. The output parameter 'new_mic_volume' was never set - instead it
     was returned as a result, causing the ADM to never update the
     analog mic gain
     (https://cs.chromium.org/chromium/src/third_party/webrtc/voice_engine/voe_base_impl.cc?q=voe_base_impl.cc&dr&l=100).

Besides this, tests are updated, and some dead code is removed which
was found in the process.

Bug: webrtc:4690, webrtc:8591
Change-Id: I789d5296bf5efb7299a5ee05a4f3ce6abf9124b2
Reviewed-on: https://webrtc-review.googlesource.com/26681
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21301}
diff --git a/audio/BUILD.gn b/audio/BUILD.gn
index 65e3050..8459385 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -20,8 +20,8 @@
     "audio_send_stream.h",
     "audio_state.cc",
     "audio_state.h",
-    "audio_transport_proxy.cc",
-    "audio_transport_proxy.h",
+    "audio_transport_impl.cc",
+    "audio_transport_impl.h",
     "conversion.h",
     "null_audio_poller.cc",
     "null_audio_poller.h",
@@ -61,6 +61,8 @@
     "../system_wrappers",
     "../system_wrappers:field_trial_api",
     "../voice_engine",
+    "../voice_engine:audio_level",
+    "utility:audio_frame_operations",
   ]
 }
 if (rtc_include_tests) {
@@ -99,6 +101,7 @@
       ":audio",
       ":audio_end_to_end_test",
       "../api:mock_audio_mixer",
+      "../call:mock_call_interfaces",
       "../call:mock_rtp_interfaces",
       "../call:rtp_interfaces",
       "../call:rtp_receiver",