Update a suppression (and add a comment) for crbug.com/20504.

chrome/browser/browser_main_mac.mm

Index: chrome/browser/browser_main_mac.mm
diff --git a/chrome/browser/browser_main_mac.mm b/chrome/browser/browser_main_mac.mm
index 0fd8fb9dec2af4d71105edfa03f4e9992d98fa65..49dc4783bde32a7d8676d05291da275f1e6d8f64 100644
--- a/chrome/browser/browser_main_mac.mm
+++ b/chrome/browser/browser_main_mac.mm
@@ -102,6 +102,8 @@ class BrowserMainPartsMac : public BrowserMainPartsPosix {
     scoped_nsobject<NSNib>
         nib([[NSNib alloc] initWithNibNamed:@"MainMenu"
                                      bundle:mac_util::MainAppBundle()]);
+    // TODO(viettrungluu): crbug.com/20504 - This currently leaks, so if you
+    // change this, you'll probably need to change the Valgrind suppression.
     [nib instantiateNibWithOwner:NSApp topLevelObjects:nil];
     // Make sure the app controller has been created.
     DCHECK([NSApp delegate]);