[Sync] Avoid leaking in WeakHandle even when the owner thread is gone

tools/heapcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or 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 (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 1675 matching lines @@
    fun:views::View::*
 }
 # This one is the most scary
 {
    bug_90013_e
    Heapcheck:Leak
    ...
    fun:??
 }
 {
-   bug_91465_a
-   Heapcheck:Leak
-   fun:base::internal::WeakReferenceOwner::GetRef
-   fun:base::WeakPtrFactory::GetWeakPtr
-   fun:sync_api::SyncManager::SyncInternal::Init
-}
-{
-   bug_91465_b
-   Heapcheck:Leak
-   fun:*WeakHandleCore
-   fun:*WeakHandle
-   fun:sync_api::SyncManager::SyncInternal::Init
-}
-{
    bug_91491
    Heapcheck:Leak
    ...
    fun:CrxUpdateService::ProcessPendingItems
 }