[base] Make PostTask destination explicit in /chromeos/dbus

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=stevenjb@chromium.org

Bug: 1026641
Change-Id: I086268457874e00af01eb548db5322ee9cf2402a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215916
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772762}

chromeos/dbus/pipe_reader_unittest.cc

@@ -16,7 +16,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/optional.h"
 #include "base/run_loop.h"
-#include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/task_runner.h"
 #include "base/test/task_environment.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -74,8 +74,8 @@ TEST_F(PipeReaderTest, SmallData) {
   base::Optional<std::string> output;
   base::ScopedFD write_fd =
       reader->StartIO(base::BindOnce(&CopyResult, &run_loop, &output));
-  base::PostTask(FROM_HERE,
-                 base::BindOnce(&WriteData, std::move(write_fd), kSmallData));
+  base::ThreadPool::PostTask(
+      FROM_HERE, base::BindOnce(&WriteData, std::move(write_fd), kSmallData));
   run_loop.Run();
   EXPECT_EQ(std::string(kSmallData), output);
 }
@@ -89,8 +89,8 @@ TEST_F(PipeReaderTest, LargeData) {
   base::Optional<std::string> output;
   base::ScopedFD write_fd =
       reader->StartIO(base::BindOnce(&CopyResult, &run_loop, &output));
-  base::PostTask(FROM_HERE,
-                 base::BindOnce(&WriteData, std::move(write_fd), large_data));
+  base::ThreadPool::PostTask(
+      FROM_HERE, base::BindOnce(&WriteData, std::move(write_fd), large_data));
   run_loop.Run();
   EXPECT_EQ(large_data, output);
 }