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

tools/valgrind/memcheck/suppressions_mac.txt

 # There are three kinds of suppressions in this file:
 # 1. Third party stuff we have no control over.
 #
 # 2. Intentional unit test errors, stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing.
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system.
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 322 matching lines @@
    Memcheck:Leak
    fun:malloc_zone_calloc
    fun:_internal_class_createInstanceFromZone
    fun:+[NSObject allocWithZone:]
    ...
    fun:-[NSCustomObject nibInstantiate]
    fun:-[NSIBObjectData instantiateObject:]
    fun:-[NSIBObjectData nibInstantiateWithOwner:topLevelObjects:]
    fun:-[NSNib instantiateNibWithExternalNameTable:]
    fun:-[NSNib instantiateNibWithOwner:topLevelObjects:]
-   fun:_Z29WillInitializeMainMessageLoopRK18MainFunctionParams
+   fun:_ZN19BrowserMainPartsMac23PreMainMessageLoopStartEv
 }
 {
    bug_20582
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN4base19MessagePumpLibevent19WatchFileDescriptorEibNS0_4ModeEPNS0_21FileDescriptorWatcherEPNS0_7WatcherE
    fun:_ZN16MessageLoopForIO19WatchFileDescriptorEibNS_4ModeEPN4base19MessagePumpLibevent21FileDescriptorWatcherEPNS2_7WatcherE
    fun:_ZN3IPC7Channel11ChannelImpl23ProcessOutgoingMessagesEv
 }
 {
@@ skipping 1230 matching lines @@
    fun:_ZN6Pickle8FindNextEmPKcS1_
    fun:_ZN3IPC7Message8FindNextEPKcS2_
    fun:_ZN3IPC7Channel11ChannelImpl23ProcessIncomingMessagesEv
    fun:_ZN3IPC7Channel11ChannelImpl28OnFileCanReadWithoutBlockingEi
    fun:_ZN4base19MessagePumpLibevent21FileDescriptorWatcher28OnFileCanReadWithoutBlockingEiPS0_
    fun:event_process_active
    fun:event_base_loop
    fun:_ZN4base19MessagePumpLibevent3RunEPNS_11MessagePump8DelegateE
    fun:_ZN11MessageLoop11RunInternalEv
 }