cros: Update virtual audio label name to Built-in mic/speaker

Changes:
As from discussion with UX writer, updating the label name here.

BUG=658048
TEST=none
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

Review-Url: https://codereview.chromium.org/2847173003
Cr-Commit-Position: refs/heads/master@{#468248}

media/audio/cras/audio_manager_cras.cc

@@ -50,8 +50,8 @@ const int kDefaultInputBufferSize = 1024;
 const char kBeamformingOnDeviceId[] = "default-beamforming-on";
 const char kBeamformingOffDeviceId[] = "default-beamforming-off";
 
-const char kInternalInputDevice[] = "Built-in-mic";
-const char kInternalOutputDevice[] = "Built-in-speaker";
+const char kInternalInputDevice[] = "Built-in mic";
+const char kInternalOutputDevice[] = "Built-in speaker";
 
 enum CrosBeamformingDeviceState {
   BEAMFORMING_DEFAULT_ENABLED = 0,