Add a new audio extension event OnLevelChanged.

extensions/shell/browser/shell_audio_controller_chromeos.cc

Index: extensions/shell/browser/shell_audio_controller_chromeos.cc
diff --git a/extensions/shell/browser/shell_audio_controller_chromeos.cc b/extensions/shell/browser/shell_audio_controller_chromeos.cc
index 760da0502650e1639485fdf6725b44f7221fc2f0..66a599df8ed89fa3c5d5e95974533f18a2c27634 100644
--- a/extensions/shell/browser/shell_audio_controller_chromeos.cc
+++ b/extensions/shell/browser/shell_audio_controller_chromeos.cc
@@ -15,7 +15,7 @@ namespace {
 // Returns a pointer to the device in |devices| with ID |node_id|, or NULL if it
 // isn't present.
 const chromeos::AudioDevice* GetDevice(const chromeos::AudioDeviceList& devices,
-                                       uint64 node_id) {
+                                       uint64_t node_id) {
   for (chromeos::AudioDeviceList::const_iterator it = devices.begin();
        it != devices.end(); ++it) {
     if (it->id == node_id)
@@ -35,11 +35,15 @@ ShellAudioController::~ShellAudioController() {
   chromeos::CrasAudioHandler::Get()->RemoveAudioObserver(this);
 }
 
-void ShellAudioController::OnOutputVolumeChanged() {}
+void ShellAudioController::OnOutputNodeVolumeChanged(uint64_t /* node_id */,
+                                                     int /* volume */) {

[asargent_no_longer_on_chrome, 2015/03/17 16:21:17]

nit: why use comments instead of just using parameter names? (here and below)

[jennyz, 2015/03/17 18:17:28]

I think this is a convention we used to indicate the parameters are not used in
the code.

[asargent_no_longer_on_chrome, 2015/03/18 00:39:25]

Interesting - I don't think I've seen this pattern used before in chrome code.
Do you know if this is explicitly allowed or disallowed in the style guide? Can
you point to some other examples of this? 

[jennyz, 2015/03/18 18:24:36]

http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Function_Decl...
At the end of the section, it mentions the following with an example.
If some parameters are unused, comment out the variable name in the function
definition.

+}
 
 void ShellAudioController::OnOutputMuteChanged() {}
 
-void ShellAudioController::OnInputGainChanged() {}
+void ShellAudioController::OnInputNodeGainChanged(uint64_t /* node_id */,
+                                                  int /* gain */) {
+}
 
 void ShellAudioController::OnInputMuteChanged() {}
 
@@ -58,7 +62,7 @@ void ShellAudioController::ActivateDevices() {
   handler->GetAudioDevices(&devices);
   sort(devices.begin(), devices.end(), chromeos::AudioDeviceCompare());
 
-  uint64 best_input = 0, best_output = 0;
+  uint64_t best_input = 0, best_output = 0;
   for (chromeos::AudioDeviceList::const_reverse_iterator it = devices.rbegin();
        it != devices.rend() && (!best_input || !best_output); ++it) {
     // TODO(derat): Need to check |plugged_time|?