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

tools/valgrind/memcheck/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 # running overnight and removing outdated bugs/suppressions.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 5161 matching lines @@
 {
    bug_91462
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN12browser_sync8internal14WeakHandleCoreINS_14JsEventHandlerEEC1ERKN4base7WeakPtrIS2_EE
    fun:_ZN12browser_sync10WeakHandleINS_14JsEventHandlerEEC1ERKN4base7WeakPtrIS1_EE
    fun:_ZN12browser_sync18MockJsEventHandler12AsWeakHandleEv
    fun:_ZN12browser_sync*JsSyncManagerObserverTestC2Ev
 }
 {
-   bug_91465
-   Memcheck:Leak
-   fun:_Znw*
-   fun:_ZN12browser_sync8internal14WeakHandleCoreINS_9JsBackendEEC1ERKN4base7WeakPtrIS2_EE
-   fun:_ZN12browser_sync10WeakHandleINS_9JsBackendEEC1ERKN4base7WeakPtrIS1_EE
-   fun:_ZN8sync_api11SyncManager12SyncInternal4InitERK8FilePathRKN12browser_sync10WeakHandleINS5_14JsEventHandlerEEERKSsibPNS_23HttpPostProviderFactoryEPNS5_24ModelSafeWorkerRegistrarESC_RKNS_15SyncCredentialsEPN13sync_notifier12SyncNotifierESC_b
-}
-{
    bug_91486_a
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:_ZN12*ReadManifestERK8FilePath
    fun:_ZN17ComponentUnpacker*ERKSt6vectorIhSaIhEERK8FilePathP18ComponentInstaller
 }
 {
    bug_91486_b
    Memcheck:Free
@@ skipping 33 matching lines @@
 }
 {
    bug_todo_getdelim2
    Memcheck:Leak
    fun:malloc
    fun:getdelim
    obj:/lib/libselinux.so.1
    obj:/lib/libselinux.so.1
    obj:/lib/libselinux.so.1
 }