Mac File Dialogs: Fix crash due to adding nil value to an NSMutableSet.

ui/shell_dialogs/select_file_dialog_mac.mm

Index: ui/shell_dialogs/select_file_dialog_mac.mm
diff --git a/ui/shell_dialogs/select_file_dialog_mac.mm b/ui/shell_dialogs/select_file_dialog_mac.mm
index 1ac812236305e9eb72ef14efe0f4069354f70c1e..bf5a862562609991148364ac8a6bde4d753916d9 100644
--- a/ui/shell_dialogs/select_file_dialog_mac.mm
+++ b/ui/shell_dialogs/select_file_dialog_mac.mm
@@ -310,8 +310,11 @@ - (void)popupAction:(id)sender;
       if (ext == default_extension)
         default_extension_index = i;
 
+      // Crash reports suggest that CreateUTIFromExtension may return nil. Hence
+      // we nil check before adding to |file_type_set|. See crbug.com/630101.
       base::ScopedCFTypeRef<CFStringRef> uti(CreateUTIFromExtension(ext));
-      [file_type_set addObject:base::mac::CFToNSCast(uti.get())];
+      if (uti)
+        [file_type_set addObject:base::mac::CFToNSCast(uti.get())];
 
       // Always allow the extension itself, in case the UTI doesn't map
       // back to the original extension correctly. This occurs with dynamic