[base] Make PostTask destination explicit in /chromeos/components/nearby

Task APIs v3 will all have an explicit destination. In the past
"default traits" meant ThreadPool. These CLs are thus a no-op
except for the few callers that clearly wanted to post a QuitClosure()
on the current thread (posting a QuitClosure() to a parallel task will
merely bounce it back to the current thread anyways). Those few callers
were migrated to ThreadTaskRunnerHandle::Get().

These changes are thus required to phase out task APIs v2 (post_task.h)
Design doc: https://docs.google.com/document/d/1tssusPykvx3g0gvbvU4HxGyn3MjJlIylnsH13-Tv6s4/edit

This specific change is a subset of
https://chromium-review.googlesource.com/c/chromium/src/+/2216733

This CL was uploaded by git cl split.

R=hansberry@chromium.org

Bug: 1026641
Change-Id: If8c3e0326a77e5ffca7f6e885cbda1bd37c0b3d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216735
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772272}

chromeos/components/nearby/condition_variable_impl_unittest.cc

@@ -8,7 +8,7 @@
 
 #include "base/bind.h"
 #include "base/single_thread_task_runner.h"
-#include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/test/gtest_util.h"
 #include "base/test/task_environment.h"
 #include "base/test/test_timeouts.h"
@@ -99,7 +99,7 @@ class ConditionVariableImplTest : public testing::Test {
   FakeLock* fake_lock() { return fake_lock_.get(); }
 
   void WaitOnConditionVariableFromParallelSequence(bool should_succeed) {
-    base::PostTask(
+    base::ThreadPool::PostTask(
         FROM_HERE,
         base::BindOnce(&ConditionVariableImplTest::WaitOnConditionVariable,
                        base::Unretained(this), should_succeed));