Remove suppression for leak that was fixed in r174531.

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 # running overnight and removing outdated bugs/suppressions.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 6322 matching lines @@
    fun:_ZN4base13WaitableEvent4WaitEv
    fun:_ZN4base12SimpleThread5StartEv
    fun:_ZN4base19SequencedWorkerPool6WorkerC1ERK13scoped_refptrIS0_EiRKSs
    fun:_ZN4base19SequencedWorkerPool5Inner30FinishStartingAdditionalThreadEi
    fun:_ZN4base19SequencedWorkerPool5Inner10ThreadLoopEPNS0_6WorkerE
    fun:_ZN4base19SequencedWorkerPool6Worker3RunEv
    fun:_ZN4base12SimpleThread10ThreadMainEv
    fun:_ZN4base12_GLOBAL__N_110ThreadFuncEPv
 }
 {
-   bug_166977
-   Memcheck:Leak
-   fun:_Znw*
-   fun:_ZN24ChromePrefServiceBuilder17ResetDefaultStateEv
-   fun:_ZN24ChromePrefServiceBuilder17CreateChromePrefsERK8FilePathPN4base19SequencedTaskRunnerEPN6policy13PolicyServiceEP9PrefStoreb
-}
-{
    bug_167165a
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:_ZN5views13WebDialogViewC2EPN7content14BrowserContextEPN2ui17WebDialogDelegateEPNS4_28WebDialogWebContentsDelegate18WebContentsHandlerE
    fun:_ZN3ash19KeyboardOverlayViewC1EPN7content14BrowserContextEPN2ui17WebDialogDelegateEPNS4_28WebDialogWebContentsDelegate18WebContentsHandlerE
    fun:_ZN3ash50KeyboardOverlayViewTest_OpenAcceleratorsClose_Test8TestBodyEv
 }
 {
    bug_167165b
@@ skipping 63 matching lines @@
    fun:vp8e_encode
    fun:vpx_codec_encode
 }
 {
    bug_167855
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN9prerender24UnitTestPrerenderManager27CreateNextPrerenderContentsERK4GURLNS_11FinalStatusE
    fun:_ZN9prerender53PrerenderTest_LinkManagerWaitToLaunchNotLaunched_Test8TestBodyEv
 }