Remove a suppression for bug 18488 since it should be fixed already.

tools/valgrind/tsan/suppressions.txt

 ############################
 # Chromium
 
 {
   bug_15147
   ThreadSanitizer:Race
   ...
   fun:*URLRequestJobTracker*
 }
 
 {
-  bug_18488
-  ThreadSanitizer:Race
-  ...
-  fun:*RefCountedBase*
-  fun:*HostResolver*
-}
-
-{
   bug_21259
   ThreadSanitizer:Race
   ...
   fun:*tracked_objects*ThreadData*StartTracking*
 }
 
 {
   bug_21468
   ThreadSanitizer:Race
   ...
@@ skipping 64 matching lines @@
   Two writes, same value (ICU gGlobalMutex, gMutexesInUse)
   ThreadSanitizer:Race
   fun:umtx_init_4_2
 }
 
 {
   Two writes, same value (ICU gHeapInUse)
   ThreadSanitizer:Race
   fun:uprv_malloc_4_2
 }