[MemSheriff] Suppress leak of BackgroundReaper in EnsureProcessTerminated.

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 # running overnight and removing outdated bugs/suppressions.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 5547 matching lines @@
    fun:_ZN9alsa_util18OpenPlaybackDeviceEPN5media11AlsaWrapperEPKcii15_snd_pcm_formati
    fun:_ZN5media19AlsaPcmOutputStream16AutoSelectDeviceEj
    fun:_ZN5media19AlsaPcmOutputStream4OpenEv
    fun:_ZN5media25AudioOutputDispatcherImpl19CreateAndOpenStreamEv
    fun:_ZN5media25AudioOutputDispatcherImpl10OpenStreamEv
    fun:_ZN5media20AudioOutputResampler10OpenStreamEv
    fun:_ZN5media16AudioOutputProxy4OpenEv
    fun:_ZN5media18AudioStreamHandler20AudioStreamContainer4PlayEv
 }
 {
+   bug_318221
+   Memcheck:Leak
+   fun:_Znw*
+   fun:_ZN4base23EnsureProcessTerminatedEi
+}
+{
    bug_321976
    Memcheck:Leak
    ...
    fun:nssList_Create
    fun:nssTrustDomain_UpdateCachedTokenCerts
 }
 {
    bug_327672
    Memcheck:Uninitialized
    fun:_ZNK7blink18StyleSheetContents11isCacheableEv
@@ skipping 674 matching lines @@
 }
 {
    bug_394624
    Memcheck:Leak
    ...
    fun:_ZN3net24URLRequestJobFactoryImpl18SetProtocolHandlerERKSsPNS_20URLRequestJobFactory15ProtocolHandlerE
    ...
    fun:_ZN7content28ShellURLRequestContextGetter20GetURLRequestContextEv
    fun:_ZN7content21ChromeAppCacheService20InitializeOnIOThreadERKN4base8FilePathEPNS_15ResourceContextEPN3net23URLRequestContextGetterE13scoped_refptrIN5quota20SpecialStoragePolicyEE
 }