Avoid redundant calls to ScheduleWork

There's no need for ThreadControllerWithMessagePumpImpl::DoWork
to call ScheduleWork.  Typically it's called in a for loop, or
something equivalent.

Redundant calls to ScheduleWork are not free and show up on
profiles of SequenceManagerPerfTest.

Bug: 897751
Change-Id: Iafebf0b574d991ea3b89dfc101aa7326cd35b3c1
Reviewed-on: https://chromium-review.googlesource.com/c/1297137Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602332}

base/task/sequence_manager/thread_controller_with_message_pump_impl.cc

@@ -153,8 +153,9 @@ bool ThreadControllerWithMessagePumpImpl::DoWork() {
   DCHECK_GE(do_work_delay, TimeDelta());
   // Schedule a continuation.
   if (do_work_delay.is_zero()) {
-    // Need to run new work immediately.
-    pump_->ScheduleWork();
+    // Need to run new work immediately, but due to the contract of DoWork we
+    // only need to return true to ensure that happens.
+    return true;
   } else if (do_work_delay != TimeDelta::Max()) {
     // Cancels any previously scheduled delayed wake-ups.
     pump_->ScheduleDelayedWork(lazy_now.Now() + do_work_delay);