Introduce TaskRunnerHandle.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 0aa2a8eeb3e068d871b811962971ff807640994c..b70e1d27da8cf43aa19901e9a272f57f46064644 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -822,6 +822,9 @@ component("base") {
     "threading/sequenced_worker_pool.h",
     "threading/simple_thread.cc",
     "threading/simple_thread.h",
+    "threading/task_runner_handle.cc",

[fdoray, 2016/06/17 14:29:01]

Why is this in base/threading/? Could it be in base/ like task_runner.h? Or
could we move this, task_runner.h and (sequenced|single_thread)_task_runner.h in
base/task/?

IMO, base/threading/ should be only for:
- Managing threads and their properties (platform_thread.h, simple_thread.h,
sequenced_worker_pool.h, thread_id_name_manager.h, ...)
- Providing access to thread-local storage (thread_local.h, ...)
- Verifying that code runs on the appropriate thread (non_thread_safe.h,
thread_restrictions.h, ...)

[gab, 2016/06/20 19:55:35]

I agree that this is weird, I had the same thought multiple times (I hate that
task_runners are in base/ root and also find it weird for the
task_runner_handles to be in base/threading/.

If we introduce something like base/task, maybe we could put post_task.h in it
as well? Keeping base/task_scheduler for the implementation? Do you want to kick
off a discussion on this in a bug with base/ OWNERS?

+    "threading/task_runner_handle.h",
+    "threading/task_runner_handle_internal.h",
     "threading/thread.cc",
     "threading/thread.h",
     "threading/thread_checker.h",
@@ -1896,6 +1899,7 @@ test("base_unittests") {
     "threading/sequenced_task_runner_handle_unittest.cc",
     "threading/sequenced_worker_pool_unittest.cc",
     "threading/simple_thread_unittest.cc",
+    "threading/task_runner_handle_unittest.cc",
     "threading/thread_checker_unittest.cc",
     "threading/thread_collision_warner_unittest.cc",
     "threading/thread_id_name_manager_unittest.cc",