Ensure that an autorelease pool is available in worker_pool_mac's WorkerPool::PostTask

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 # running overnight and removing outdated bugs/suppressions.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 807 matching lines @@
 {
    bug_38280
    Memcheck:Leak
    fun:_Znam
    fun:_ZN2v88internalL8NewArrayIPPNS0_6ObjectEEEPT_i
    fun:_ZN2v88internal6VectorIPPNS0_6ObjectEE3NewEi
    fun:_ZN2v88internalL13CallJsBuiltinEPKcNS0_12_GLOBAL__N_116BuiltinArgumentsILNS0_21BuiltinExtraArgumentsE0EEE
    fun:_ZN2v88internalL23Builtin_Impl_ArraySliceENS0_12_GLOBAL__N_116BuiltinArgumentsILNS0_21BuiltinExtraArgumentsE0EEE
    fun:_ZN2v88internalL18Builtin_ArraySliceENS0_12_GLOBAL__N_116BuiltinArgumentsILNS0_21BuiltinExtraArgumentsE0EEE
 }
-{
-   bug_38357
-   Memcheck:Leak
-   ...
-   fun:_NSKeyValueCreateImplicitObservationInfo
-   fun:-[NSOperation init]
-   fun:-[TaskOperation initWithTask:]
-   fun:+[TaskOperation taskOperationWithTask:]
-   fun:_ZN10WorkerPool8PostTaskERKN15tracked_objects8LocationEP4Taskb
-   fun:_ZN3net16HostResolverImpl12IPv6ProbeJob5StartEv
-   fun:_ZN3net16HostResolverImpl18OnIPAddressChangedEv
-   fun:_ZN3net16HostResolverImpl16ProbeIPv6SupportEv
-   fun:_ZN12_GLOBAL__N_124CreateGlobalHostResolverEPN3net21NetworkChangeNotifierE
-}