[Sync] Fix a subtle use-after-free in Attachments.

tools/valgrind/memcheck/suppressions.txt

 # There are four 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 # running overnight and removing outdated bugs/suppressions.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 3136 matching lines @@
 {
    bug_602964
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN4base4BindIMNS_12_GLOBAL__N_121PostTaskAndReplyRelayEFvvEJNS_8internal17UnretainedWrapper*
    fun:_ZN4base8internal20PostTaskAndReplyImpl16PostTaskAndReplyERKN15tracked_objects8LocationERKNS*
    fun:_ZN4base10TaskRunner16PostTaskAndReplyERKN15tracked_objects8LocationERKNS_8CallbackIFvvELNS*
    fun:_ZN4base26PostTaskAndReplyWithResultIN3net20WifiPHYLayerProtocolES2_EEbPNS*
    fun:_ZN7metrics22NetworkMetricsProvider25ProbeWifiPHYLayerProtocolEv
 }
-{
-   bug_603275
-   Memcheck:Unaddressable
-   fun:_ZNKSs4sizeEv
-   fun:_ZSteqIcEN9__gnu_cxx11__enable_ifIXsr9__is_charIT_EE7__valueEbE6__typeERKSbIS2_St11char_traitsIS2_ESaIS2_EESA_
-   fun:_ZNKSt8equal_toISsEclERKSsS2_
-   fun:_ZNKSt8__detail15_Hash_code_baseISsSt4pairIKSsSt10unique_ptrIN6syncer24AttachmentDownloaderImpl13DownloadState*
-   fun:_ZNSt10_HashtableISsSt4pairIKSsSt10unique_ptrIN6syncer24AttachmentDownloaderImpl13DownloadStateESt14default*
-   fun:_ZN6syncer24AttachmentDownloaderImpl17OnGetTokenFailureEPKN18OAuth2TokenService7RequestERK22GoogleServiceAuthError
-   fun:_ZThn8_N6syncer24AttachmentDownloaderImpl17OnGetTokenFailureEPKN18OAuth2TokenService7RequestERK22GoogleServiceAuthError
-   fun:_ZN12_GLOBAL__N_111RequestCore28InformOwnerOnGetTokenFailureE22GoogleServiceAuthError
-}