Fix memory leak in PrerenderDispatcher.

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 # running overnight and removing outdated bugs/suppressions.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 5736 matching lines @@
    fun:_ZN45BrowserThemePackTest_CanBuildAndReadPack_Test8TestBodyEv
 }
 {
    bug_127718:
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN3gfx9JPEGCodec6DecodeEPKhm
    fun:_ZN7history33HistoryBackendTest_DeleteAll_Test8TestBodyEv
 }
 {
-   bug_127953
-   Memcheck:Leak
-   fun:_Znw*
-   fun:_ZN9prerender19PrerenderDispatcherC1Ev
-   fun:_ZN6chrome27ChromeContentRendererClient19RenderThreadStartedEv
-   fun:_ZN16RenderThreadImpl4InitEv
-}
-{
    bug_128145a
    Memcheck:Leak
    fun:_Znw*
    fun:_ZNK21WebDataServiceFactory23BuildServiceInstanceForEP7Profile
    fun:_ZN36RefcountedProfileKeyedServiceFactory20GetServiceForProfileEP7Profileb
    fun:_ZN21WebDataServiceFactory13GetForProfileEP7ProfileNS0_17ServiceAccessTypeE
    fun:_ZN12TokenService10InitializeEPKcP7Profile
    fun:_ZN13SigninManager10InitializeEP7Profile
    fun:_ZNK20SigninManagerFactory23BuildServiceInstanceForEP7Profile
    fun:_ZN26ProfileKeyedServiceFactory20GetServiceForProfileEP7Profileb
@@ skipping 43 matching lines @@
    obj:/lib/libselinux.so.1
    obj:/lib/libselinux.so.1
 }
 {
    bug_todo_grep
    Memcheck:Leak
    fun:malloc
    obj:/bin/grep
    obj:/bin/grep
 }