Adds comments to BrowserList::GetLastActive that it might return NULL...

chrome/browser/browser_list.h

Index: chrome/browser/browser_list.h
===================================================================
--- chrome/browser/browser_list.h	(revision 16943)
+++ chrome/browser/browser_list.h	(working copy)
@@ -48,6 +48,11 @@
   // Returns the Browser object whose window was most recently active.  If the
   // most recently open Browser's window was closed, returns the first Browser
   // in the list.  If no Browsers exist, returns NULL.
+  //
+  // WARNING: this is NULL until a browser becomes active. If during startup
+  // a browser does not become active (perhaps the user launches Chrome, then
+  // clicks on another app before the first browser window appears) then this
+  // returns NULL.
   static Browser* GetLastActive();
 
   // Identical in behavior to GetLastActive(), except that the most recently