Adds extension APIs of events on changing volume.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 364 matching lines @@
       {
         "id": "TextBoxDetails",
         "type": "object",
         "description": "Information about the state of a text box.",
         "properties": {
           "value": {"type": "string", "description": "The value of the text box - the entered text."},
           "isPassword": {"type": "boolean", "description": "True if this control contains password text whose contents should be obscured."},
           "selectionStart": {"type": "integer", "description": "The index of the character where the selection starts, if this control contains editable text."},
           "selectionEnd": {"type": "integer", "description": "The index of the character where the selection ends, if this control contains editable text."}
         }
+      },
+      {
+        "id": "AccessibilityVolumeInfo",
+        "type": "object",
+        "description": "Information about the volume.",
+        "properties": {
+          "volume": {"type": "double", "description": "The value of the volume percent. This must be between 0.0 and 100.0."},

[Ben Olmstead, 2011/08/30 18:12:18]

Is the value log scale or linear scale?

+          "isVolumeMuted": {"type": "boolean", "description": "True if the volume is muted."}
+        }
       }
     ],
     "functions": [
       {
         "name": "setAccessibilityEnabled",
         "type": "function",
         "description": "Enables or disables the accessibility extension api. This must be set to true before event listeners or getFocusedControl will work.",
         "parameters": [
           {
             "type": "boolean",
@@ skipping 101 matching lines @@
         "name": "onMenuClosed",
         "type": "function",
         "description": "Fired when a menu is closed.",
         "parameters": [
           {
             "$ref": "AccessibilityObject",
             "name": "menu",
             "description": "Information about the menu that was closed."
           }
         ]
+      },
+      {
+        "name": "onVolumeChanged",
+        "type": "function",
+        "description": "Fired when the volume is changed.",
+        "parameters": [
+          {
+            "$ref": "AccessibilityVolumeInfo",
+            "name": "volume",
+            "description": "Information about the current volume level and the volume being muted."

[Ben Olmstead, 2011/08/30 18:12:18]

"The current state of the system volume control, including whether it is muted."
?

+          }
+        ]
       }
     ]
   },
   {
     "namespace": "tts",
     "types": [
       {
         "id": "TtsEvent",
         "type": "object",
         "description": "An event from the TTS engine to communicate the status of an utterance.",
@@ skipping 7809 matching lines @@
             "type": "function",
             "description": "Called when the browser's passwords have been cleared.",
             "optional": true,
             "parameters": []
           }
         ]
       }
     ]
   }
 ]