Initialize the global message center instance to NULL

BUG=Uninitialized global message center instance leads to trouble when
modifying gyps.

Review URL: https://codereview.chromium.org/444463007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287763 0039d316-1c4b-4281-b951-d872f2087c98

ui/message_center/message_center.cc

@@ -12,7 +12,7 @@ namespace message_center {
 //------------------------------------------------------------------------------
 
 namespace {
-static MessageCenter* g_message_center;
+static MessageCenter* g_message_center = NULL;
 }
 
 // static