Fix memory leak in SyncableExtensionSettingsStorage.

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 # running overnight and removing outdated bugs/suppressions.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 1676 matching lines @@
    fun:sync_notifier::NonBlockingInvalidationNotifier::Core::UpdateCredentials
    fun:void DispatchToMethod
 }
 {
    bug_97511_a
    Heapcheck:Leak
    ...
    fun:ExtensionSettingsQuotaUnittest*
 }
 {
-   bug_97511_b
-   Heapcheck:Leak
-   ...
-   fun:ExtensionSettingsSyncTest*
-}
-{
    bug_97511_c
    Heapcheck:Leak
    ...
    fun:ExtensionUpdaterTest*
 }
 {
    bug_97511_d
    Heapcheck:Leak
    ...
    fun:ExtensionServiceTestSimple*
 }