Add support for a "split" incognito behavior for extensions.

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 33 matching lines @@
     ],
     "properties": {
       "lastError": {
         "type": "object",
         "optional": true,
         "description": "Set for the lifetime of a callback if an ansychronous extension api has resulted in an error. If no error has occured lastError will be <var>undefined</var>.",
         "properties": {
           "message": { "type": "string", "description": "Description of the error that has taken place." }
         }
       },
-      "inIncognitoTab": {
+      "inIncognitoContext": {
         "type": "object",
         "optional": true,
         "unprivileged": true,
-        "description": "True for content scripts running inside incognito tabs. Will be undefined for non-content scripts.",
+        "description": "True for content scripts running inside incognito tabs, and for extension pages running inside an incognito process. The latter only applies to extensions with 'split' incognito_behavior.",
         "properties": {
-          "message": { "type": "boolean", "description": "True if the script is in an incongito tab." }
+          "message": { "type": "boolean", "description": "True if the page or script is in an incongito process." }
         }
       }
     },
     "functions": [
       {
         "name": "connect",
         "type": "function",
         "unprivileged": true,
         "description": "Attempts to connect to other listeners within the extension (such as the extension's background page). This is primarily useful for content scripts connecting to their extension processes. Extensions may connect to content scripts embedded in tabs via <a href='tabs.html#method-connect'><code>chrome.tabs.connect()</code></a>.",
         "parameters": [
@@ skipping 3994 matching lines @@
       },
       {
         "name": "onDisabled",
         "description": "Fired when an app or extension has been disabled",
         "type": "function",
         "parameters": [{"name": "info", "$ref":"ExtensionInfo"}]
       }
     ]
   }
 ]