[Mac] Fix a leak in CookiesWindowControllerTest by giving BrowserTestHelper a fake IO thread.

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 # running overnight and removing outdated bugs/suppressions.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 530 matching lines @@
 }
 {
    bug_30632b
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:_ZN17PluginProcessHost4InitERK13WebPluginInfoRKSbIwSt11char_traitsIwESaIwEE
    fun:_ZN13PluginService24FindOrStartPluginProcessERK8FilePath
 }
 {
-   bug_31586a
-   Memcheck:Leak
-   fun:_Znw*
-   ...
-   fun:_ZN3net13CookieMonster20InternalInsertCookieERKSsPNS0_15CanonicalCookieEb
-   fun:_ZN3net13CookieMonster18SetCanonicalCookieEP10scoped_ptrINS0_15CanonicalCookieEERKSsRKN4base4TimeERKNS_13CookieOptionsE
-   fun:_ZN3net13CookieMonster35SetCookieWithCreationTimeAndOptionsERK4GURLRKSsRKN4base4TimeERKNS_13CookieOptionsE
-   fun:_ZN3net13CookieMonster20SetCookieWithOptionsERK4GURLRKSsRKNS_13CookieOptionsE
-   fun:_ZN3net11CookieStore9SetCookieERK4GURLRKSs
-}
-{
-   bug_31586b
-   Memcheck:Leak
-   fun:_Znw*
-   fun:_ZN14TestingProfile20CreateRequestContextEv
-   fun:_ZN12_GLOBAL__N_127CookiesWindowControllerTest5SetUpEv
-}
-{
    bug_31634a
    Memcheck:Leak
    fun:malloc_zone_malloc
    fun:_CFRuntimeCreateInstance
    fun:__CFDictionaryInit
    fun:CFDictionaryCreate
    fun:classDescription
    fun:classDescription
    fun:classDescription
    fun:propertyInfoForSelector
@@ skipping 625 matching lines @@
    fun:_ZN15AQIONodeManager13_FindAQIONodeEmbb
    fun:_ZN15AQIONodeManager12FindAQIONodeEmbb
    fun:_ZN16AudioQueueObject15DoIONodeConnectERK25AudioQueueStreamSpecifierPK27AudioStreamBasicDescriptionPK18AudioChannelLayout
    fun:_ZN16AudioQueueObject13EnqueueBufferEP16AudioQueueBufferjPK28AudioStreamPacketDescriptionjjjPK24AudioQueueParameterEventRK11AQTimeStampPS8_
    fun:AQServer_EnqueueBuffer
    fun:AudioQueueEnqueueBufferWithParameters
    fun:AudioQueueEnqueueBuffer
    fun:_ZN28PCMQueueOutAudioOutputStream5StartEPN17AudioOutputStream19AudioSourceCallbackE
    fun:_ZN17AudioRendererHost14IPCAudioSource4PlayEv
 }