Add default zoom functionality to chrome.tabs Zoom API.

chrome/common/extensions/api/tabs.json

Index: chrome/common/extensions/api/tabs.json
diff --git a/chrome/common/extensions/api/tabs.json b/chrome/common/extensions/api/tabs.json
index 4fe70c74c900b9419ff51b5ae9e51131c1d1bc0e..4ed0b123f763e4f2b774c9ab8fc6095f7623c257 100644
--- a/chrome/common/extensions/api/tabs.json
+++ b/chrome/common/extensions/api/tabs.json
@@ -68,6 +68,12 @@
                 "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,

[not at google - send to devlin, 2015/04/01 20:34:07]

Why optional?

[wjmaclean, 2015/04/01 21:23:24]

Making it not-optional requires callers to setZoomSettings(), which uses this
same struct, to specify a value for this parameter (and the value is just
ignored). I think it's best not to require input of a dummy value.

+            "description": "Used to return the default zoom level for the current tab in calls to tabs.getZoomSettings."
           }
         }
       }