Add chrome.debugger.getTargets method to discover available debug targets.

chrome/common/extensions/api/debugger.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "debugger",
     "types": [
       {
         "id": "Debuggee",
         "type": "object",
         "description": "Debuggee identifier. Either tabId or extensionId must be specified",
         "properties": {
           "tabId": { "type": "integer", "optional": true, "description": "The id of the tab which you intend to debug." },
-          "extensionId": { "type": "string", "optional": true, "description": "The id of the extension which you intend to debug. Attaching to an extension background page is only possible when 'enable-silent-debugging' flag is enabled on the target browser." }
+          "extensionId": { "type": "string", "optional": true, "description": "The id of the extension which you intend to debug. Attaching to an extension background page is only possible when 'enable-silent-debugging' flag is enabled on the target browser." },
+          "targetId": { "type": "string", "optional": true, "description": "The opaque id of the debug target." }
+        }
+      },
+      {
+        "id": "TargetInfo",
+        "type": "object",
+        "description": "Debug target information",
+        "properties": {
+          "type": {
+            "type": "string",
+            "description": "Target type.",
+            "enum" : ["page", "background_page", "other" ]
+          },
+          "id": { "type": "string", "description": "Target id." },
+          "attached": { "type": "boolean", "description": "True if debugger is already attached." },
+          "title": { "type": "string", "description": "Target page title." },
+          "url": { "type": "string", "description": "Target URL." },
+          "faviconUrl": { "type": "string", "optional": true, "description": "Target favicon URL." }
         }
       }
     ],
     "functions": [
       {
         "name": "attach",
         "type": "function",
         "description": "Attaches debugger to the given target.",
         "parameters": [
           {
@@ skipping 65 matching lines @@
                 "type": "object",
                 "name": "result",
                 "optional": true,
                 "additionalProperties": { "type": "any" },
                 "description": "JSON object with the response. Structure of the response varies depending on the method and is defined by the remote debugging protocol."
               }
             ],
             "description": "Response body. If an error occurs while posting the message, the callback will be called with no arguments and $ref:runtime.lastError will be set to the error message."
           }
         ]
+      },
+      {
+        "name": "getTargets",
+        "type": "function",
+        "description": "Returns the list of available debug targets.",
+        "parameters": [
+          {
+            "type": "function",
+            "name": "callback",
+            "parameters": [
+              {
+                "type": "array",
+                "name": "result",
+                "items": {"$ref": "TargetInfo"},
+                "description": "Array of TargetInfo objects corresponding to the available debug targets."
+              }
+            ]
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onEvent",
         "type": "function",
         "description": "Fired whenever debugging target issues instrumentation event.",
         "parameters": [
           {
             "$ref": "Debuggee",
@@ skipping 28 matching lines @@
             "type": "string",
             "name": "reason",
             "description": "Connection termination reason.",
             "enum": [ "target_closed", "canceled_by_user", "replaced_with_devtools" ]
           }
         ]
       }
     ]
   }
 ]