Add global chrome object through window

It allows to add a check for existence of 'chrome' object.
Without it the strict mode can complain about undefined variable.

chrome/chrome.d.ts

@@ -5,6 +5,13 @@
 
 /// <reference path='../webrtc/MediaStream.d.ts'/>
 
+////////////////////
+// Global object
+////////////////////
+interface Window {
+    chrome: typeof chrome;
+}
+
 ////////////////////
 // Alarms
 ////////////////////