Cleanup of iOS AudioDevice implementation

TBR=tkchin
BUG=webrtc:4789
TEST=modules_unittests --gtest_filter=AudioDeviceTest* and AppRTCDemo

Review URL: https://codereview.webrtc.org/1206783002 .

Cr-Commit-Position: refs/heads/master@{#9578}
diff --git a/webrtc/modules/audio_device/include/audio_device.h b/webrtc/modules/audio_device/include/audio_device.h
index 2f0c6b5..1826e0e 100644
--- a/webrtc/modules/audio_device/include/audio_device.h
+++ b/webrtc/modules/audio_device/include/audio_device.h
@@ -202,8 +202,17 @@
   // Don't use.
   virtual bool BuiltInAECIsEnabled() const { return false; }
 
+  // Only supported on iOS.
+  // TODO(henrika): Make pure virtual after updating Chromium.
+  virtual int GetPlayoutAudioParameters(AudioParameters* params) const {
+    return -1;
+  }
+  virtual int GetRecordAudioParameters(AudioParameters* params) const {
+    return -1;
+  }
+
  protected:
-  virtual ~AudioDeviceModule() {};
+  virtual ~AudioDeviceModule() {}
 };
 
 AudioDeviceModule* CreateAudioDeviceModule(