Report default audio input device name on CrOS

This adds an implementation of the GetDefaultInputDeviceID()
method for AudioManagerCras.

Bug: 780492
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: Icbd00994c5b4d3008946b59eabadca7685aed9aa
Reviewed-on: https://chromium-review.googlesource.com/810767Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522116}

media/audio/cras/audio_manager_cras.cc

@@ -225,6 +225,11 @@ std::string AudioManagerCras::GetAssociatedOutputDeviceID(
              : base::Uint64ToString(output_device_it->id);
 }
 
+std::string AudioManagerCras::GetDefaultInputDeviceID() {
+  DCHECK(GetTaskRunner()->BelongsToCurrentThread());
+  return base::Uint64ToString(GetPrimaryActiveInputNode());
+}
+
 std::string AudioManagerCras::GetDefaultOutputDeviceID() {
   DCHECK(GetTaskRunner()->BelongsToCurrentThread());
   return base::Uint64ToString(GetPrimaryActiveOutputNode());

media/audio/cras/audio_manager_cras.h

@@ -34,6 +34,7 @@ class MEDIA_EXPORT AudioManagerCras : public AudioManagerBase {
       const std::string& device_id) override;
   std::string GetAssociatedOutputDeviceID(
       const std::string& input_device_id) override;
+  std::string GetDefaultInputDeviceID() override;
   std::string GetDefaultOutputDeviceID() override;
   std::string GetGroupIDOutput(const std::string& output_device_id) override;
   std::string GetGroupIDInput(const std::string& input_device_id) override;