Simplification and refactoring of the AudioBuffer code

This CL performs a major refactoring and simplification
of the AudioBuffer code that.
-Removes 7 of the 9 internal buffers of the AudioBuffer.
-Avoids the implicit copying required to keep the
 internal buffers in sync.
-Removes all code relating to handling of fixed-point
 sample data in the AudioBuffer.
-Changes the naming of the class methods to reflect
 that only floating point is handled.
-Corrects some bugs in the code.
-Extends the handling of internal downmixing to be
 more generic.

Bug: webrtc:10882
Change-Id: I12c8af156fbe366b154744a0a1b3d926bf7be572
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149828
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28928}

modules/audio_processing/audio_buffer_unittest.cc

@@ -16,7 +16,7 @@ namespace webrtc {
 
 namespace {
 
-const size_t kNumFrames = 480u;
+const size_t kSampleRateHz = 48000u;
 const size_t kStereo = 2u;
 const size_t kMono = 1u;
 
@@ -27,17 +27,17 @@ void ExpectNumChannels(const AudioBuffer& ab, size_t num_channels) {
 }  // namespace
 
 TEST(AudioBufferTest, SetNumChannelsSetsChannelBuffersNumChannels) {
-  AudioBuffer ab(kNumFrames, kStereo, kNumFrames, kStereo, kNumFrames);
+  AudioBuffer ab(kSampleRateHz, kStereo, kSampleRateHz, kStereo, kSampleRateHz);
   ExpectNumChannels(ab, kStereo);
-  ab.set_num_channels(kMono);
+  ab.set_num_channels(1);
   ExpectNumChannels(ab, kMono);
-  ab.InitForNewData();
+  ab.RestoreNumChannels();
   ExpectNumChannels(ab, kStereo);
 }
 
 #if RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
 TEST(AudioBufferTest, SetNumChannelsDeathTest) {
-  AudioBuffer ab(kNumFrames, kMono, kNumFrames, kMono, kNumFrames);
+  AudioBuffer ab(kSampleRateHz, kMono, kSampleRateHz, kMono, kSampleRateHz);
   EXPECT_DEATH(ab.set_num_channels(kStereo), "num_channels");
 }
 #endif