Use ChromeThread::IO instead of virtual set_request_context

tools/valgrind/memcheck/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 # running overnight and removing outdated bugs/suppressions.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 4132 matching lines @@
    fun:_ZNKSt8_Rb_treeIjSt4pairIKjP12ObserverListI20NotificationObserverLb0EEESt10_Select1stIS6_ESt4lessIjESaIS6_EE8_M_beginEv
    fun:_ZNKSt8_Rb_treeIjSt4pairIKjP12ObserverListI20NotificationObserverLb0EEESt10_Select1stIS6_ESt4lessIjESaIS6_EE4findERS1_
    fun:_ZNKSt3mapIjP12ObserverListI20NotificationObserverLb0EESt4lessIjESaISt4pairIKjS3_EEE4findERS7_
    fun:_ZN19NotificationService6HasKeyERKSt3mapIjP12ObserverListI20NotificationObserverLb0EESt4lessIjESaISt4pairIKjS4_EEERK18NotificationSource
    fun:_ZN19NotificationService6NotifyE16NotificationTypeRK18NotificationSourceRK19NotificationDetails
    fun:_ZN23ChromeURLRequestContextD0Ev
    ...
    fun:_ZN17URLRequestHttpJobD0Ev
 }
 {
-   bug_52826
-   Memcheck:Leak
-   fun:_Znw*
-   fun:_ZN14TestingProfile20CreateRequestContextEv
-   fun:_ZN20GoogleURLTrackerTest20CreateRequestContextEv
-}
-{
    bug_52831
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:*InotifyReaderTask3RunEv
    fun:_ZN11MessageLoop7RunTaskEP4Task
    fun:_ZN11MessageLoop21DeferOrRunPendingTaskERKNS_11PendingTaskE
    fun:_ZN11MessageLoop6DoWorkEv
    fun:_ZN4base18MessagePumpDefault3RunEPNS_11MessagePump8DelegateE
    fun:_ZN11MessageLoop11RunInternalEv
@@ skipping 51 matching lines @@
    fun:_ZN3IPC16MessageWithTupleI6Tuple1IiEE8DispatchI9GpuThreadMS5_FviEEEbPKNS_7MessageEPT_T0_
    fun:_ZN9GpuThread24OnControlMessageReceivedERKN3IPC7MessageE
 }
 {
    bug_53044
    Memcheck:Leak
    fun:malloc
    ...
    fun:_ZN24mozilla_security_manager19nsPKCS12Blob_ImportEPKcjRKSbItN4base20string16_char_traitsESaItEE
 }