[TaskScheduler]: Remove unecessary ScopedAllowWait in MessagePump.

SyncWaiter::sw needs to be declared only used while idle in WaitableEvent
to avoid being considered blocking.

Bug: 847501
Change-Id: I9a70aa28e74dbaae5f4b1b8aa66bb27ed0b5ee28
Reviewed-on: https://chromium-review.googlesource.com/c/1349730Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611146}

base/message_loop/message_pump_default.cc

@@ -54,7 +54,6 @@ void MessagePumpDefault::Run(Delegate* delegate) {
     if (did_work)
       continue;
 
-    ThreadRestrictions::ScopedAllowWait allow_wait;
     if (delayed_work_time_.is_null()) {
       event_.Wait();
     } else {

base/synchronization/waitable_event_posix.cc

@@ -189,6 +189,8 @@ bool WaitableEvent::TimedWaitUntil(const TimeTicks& end_time) {
   }
 
   SyncWaiter sw;
+  if (!waiting_is_blocking_)
+    sw.cv()->declare_only_used_while_idle();
   sw.lock()->Acquire();
 
   Enqueue(&sw);