Implement chrome.extension.connectExternal and fix various API inconsistencies.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "Port",
         "type": "object",
         "description": "An object which allows two way communication with other pages.",
         "properties": {
           "name": {"type": "string"},
           "onDisconnect": {"type": "object"},
           "onMessage": {"type": "object"},
           "postMessage": {"type": "function"},
-          "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present on ports passed to onConnect listeners when the connection was opened from a tab or content script."}
+          "sender": {
+            "type": "object",
+            "optional": true,
+            "description": "This property will <b>only</b> be present on ports passed to onConnect/onConnectExternal listeners.",
+            "properties": {
+              "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
+              "id": {"type": "string", "description": "The extension ID of the extension that opened the connection."}
+            }
+          }
         }
       }
     ],
     "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." }
         }
       }
     },
     "functions": [
       {
         "name": "connect",
         "type": "function",
         "description": "Attempts to connect to other listeners within the extension (listeners may be toolstrips or 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='broken'><var>chrome.tabs.connectToTab</var></a>.",
         "parameters": [
+          {"type": "string", "name": "extensionId", "optional": true, "description": "The extension ID of the extension you want to connect to. If omitted, default is your own extension."},
           {
             "type": "object",
             "name": "connectInfo",
             "properties": {
               "name": { "type": "string", "optional": true, "description": "Will be passed into onConnect for extension processes that are listening for the connection event." }
             },
             "optional": true
           }
         ],
         "returns": {
@@ skipping 65 matching lines @@
       }
     ],
     "events": [
       {
         "name": "onConnect",
         "type": "function",
         "description": "Fired when a connection is made from either an extension process or a content script.",
         "parameters": [
           {"$ref": "Port", "name": "port"}
         ]
+      },
+      {
+        "name": "onConnectExternal",
+        "type": "function",
+        "description": "Fired when a connection is made from another extension.",
+        "parameters": [
+          {"$ref": "Port", "name": "port"}
+        ]
       }
     ]
   },
   {
     "namespace": "windows",
     "types": [
       {
         "id": "Window",
         "type": "object",
         "properties": {
@@ skipping 1130 matching lines @@
         "type": "function",
         "description": "Logs a message during internal unit testing.",
         "parameters": [
           {"type": "string", "name": "message"}
         ]
       }
     ],
     "events": []
   }
 ]