Mark EventToTypeString with ALLOW_UNUSED to fix the Official gtk build.

It's only used inside an UNSHIPPED_TRACE_EVENT1(), which causes:

../../base/message_loop/message_pump_gtk.cc:17:13: error: unused function 'EventToTypeString' [-Werror,-Wunused-function]

Review URL: https://codereview.chromium.org/131323005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244283 0039d316-1c4b-4281-b951-d872f2087c98

base/message_loop/message_pump_gtk.cc

@@ -14,7 +14,7 @@ namespace base {
 
 namespace {
 
-const char* EventToTypeString(const GdkEvent* event) {
+ALLOW_UNUSED const char* EventToTypeString(const GdkEvent* event) {
   switch (event->type) {
     case GDK_NOTHING:           return "GDK_NOTHING";
     case GDK_DELETE:            return "GDK_DELETE";