Move std::chrono using statements to util/

This patch moves openscreen::<time type> using statements to be part of
a new util/ header, chrono_helpers.h. Some helpful convenience
functions are provided for doing conversions, and a new conversion
method, to_duration, is added to the Clock implementation.

Change-Id: I5a153c4da5266bceea97de0cad00a71a739f71ca
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2222684
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Ryan Keane <rwkeane@google.com>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
diff --git a/platform/impl/task_runner.h b/platform/impl/task_runner.h
index a44e1bd..65dfbf5 100644
--- a/platform/impl/task_runner.h
+++ b/platform/impl/task_runner.h
@@ -8,7 +8,7 @@
 #include <condition_variable>  // NOLINT
 #include <map>
 #include <memory>
-#include <mutex>  // NOLINT
+#include <mutex>
 #include <thread>
 #include <utility>
 #include <vector>