Fix mem leaks in DomStorageContextTest.

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 # running overnight and removing outdated bugs/suppressions.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 6065 matching lines @@
    fun:_ZN77GeolocationInfoBarQueueControllerTests_OneObservationPerInfoBarCancelled_Test8TestBodyEv
 }
 {
   bug_156705
   Memcheck:Leak
   fun:_Znw*
   fun:_ZN33GeolocationInfoBarQueueController21CreateInfoBarDelegateEP16InfoBarTabHelperPS_iiiRK4GURLRKSs
   fun:_ZN33GeolocationInfoBarQueueController17ShowQueuedInfoBarEiiP16InfoBarTabHelper
 }
 {
-  bug_156784
-  Memcheck:Leak
-  ...
-  fun:_ZN11dom_storage47DomStorageContextTest_DeleteSessionStorage_Test8TestBodyEv
-}
-{
   bug_156829
   Memcheck:Leak
   fun:_Znw*
   fun:_ZN12media_stream23AudioInputDeviceManager23EnumerateOnDeviceThreadEv
 }