Drain input audio device buffer completely on Windows.

* Move the code pulling and pushing data to its own function.
* Add a loop that runs until the audio client reports empty buffer.
* Remove unnecessary GetVolume() call; it's overwritten by GetAgcVolume() call later.

Bug: 637558
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ie06d862592143d76508a7d4d709efd42c83a1d9c
Reviewed-on: https://chromium-review.googlesource.com/931554
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538859}

media/audio/win/audio_low_latency_input_win.h

@@ -119,6 +119,9 @@ class MEDIA_EXPORT WASAPIAudioInputStream
   // DelegateSimpleThread::Delegate implementation.
   void Run() override;
 
+  // Pulls capture data from the endpoint device and pushes it to the sink.
+  void PullCaptureDataAndPushToSink();
+
   // Issues the OnError() callback to the |sink_|.
   void HandleError(HRESULT err);