Suppress Uninitialized access in ExtensionManagement::Refresh

BUG=418234
TBR=miu@chromium.org

Review URL: https://codereview.chromium.org/608063002

Cr-Commit-Position: refs/heads/master@{#297028}

tools/valgrind/memcheck/suppressions.txt

@@ -3453,3 +3453,9 @@
    fun:_ZN4base19SequencedWorkerPoolC1EmRKSs
    fun:_ZN16sync_file_system13drive_backend14SyncEngineTest5SetUpEv
 }
+{
+   bug_418234
+   Memcheck:Uninitialized
+   fun:_ZN10extensions19ExtensionManagement7RefreshEv
+   fun:_ZN10extensions19ExtensionManagement22OnExtensionPrefChangedEv
+}