Add default zoom functionality to chrome.tabs Zoom API.

chrome/common/extensions/api/tabs.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": "tabs",
     "description": "Use the <code>chrome.tabs</code> API to interact with the browser's tab system. You can use this API to create, modify, and rearrange tabs in the browser.",
     "types": [
       {
@@ skipping 50 matching lines @@
             "enum": [
               {
                 "name": "per-origin",
                 "description": "Zoom changes will persist in the zoomed page's origin, i.e. all other tabs navigated to that same origin will be zoomed as well. Moreover, <code>per-origin</code> zoom changes are saved with the origin, meaning that when navigating to other pages in the same origin, they will all be zoomed to the same zoom factor. The <code>per-origin</code> scope is only available in the <code>automatic</code> mode."
               },
               {
                 "name": "per-tab",
                 "description": "Zoom changes only take effect in this tab, and zoom changes in other tabs will not affect the zooming of this tab. Also, <code>per-tab</code> zoom changes are reset on navigation; navigating a tab will always load pages with their <code>per-origin</code> zoom factors."
               }
             ]
+          },
+          "default_zoom_factor": {
+            "type": "number",
+            "name": "defaultZoomFactor",
+            "optional": true,
+            "description": "Used to return the default zoom level for the current tab in calls to tabs.getZoomSettings."
           }
         }
       }
     ],
     "functions": [
       {
         "name": "get",
         "type": "function",
         "description": "Retrieves details about the specified tab.",
         "parameters": [
@@ skipping 1001 matching lines @@
             "tabId": {"type": "integer", "minimum": 0},
             "oldZoomFactor": {"type": "number"},
             "newZoomFactor": {"type": "number"},
             "zoomSettings": {"$ref": "ZoomSettings"}
           }
         }]
       }
     ]
   }
 ]