Flatten namespace for platform/ code: opescreen::platform → openscreen

Removes the nested "platform" namespace in the platform code. Fixes all
typenames (fixing/removing using namespace statements, removing
unnecessary namespace qualifiers, etc.).

Also, removed the "using UdpSocketUniquePtr = std::unique_ptr<>" since
the alias is no longer necessary (the smart pointer no longer does
anything special at delete time).

Bug: openscreen:90
Change-Id: I0e8589282b4e42530c8c1c95115078d83e3e5f16
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/1948540
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
diff --git a/platform/impl/task_runner.h b/platform/impl/task_runner.h
index 35da181..bbdd1cc 100644
--- a/platform/impl/task_runner.h
+++ b/platform/impl/task_runner.h
@@ -22,7 +22,6 @@
 #include "util/trace_logging.h"
 
 namespace openscreen {
-namespace platform {
 
 class TaskRunnerImpl final : public TaskRunner {
  public:
@@ -49,7 +48,7 @@
   };
 
   explicit TaskRunnerImpl(
-      platform::ClockNowFunctionPtr now_function,
+      ClockNowFunctionPtr now_function,
       TaskWaiter* event_waiter = nullptr,
       Clock::duration waiter_timeout = std::chrono::milliseconds(100));
 
@@ -111,7 +110,7 @@
   // transferred.
   bool GrabMoreRunnableTasks();
 
-  const platform::ClockNowFunctionPtr now_function_;
+  const ClockNowFunctionPtr now_function_;
 
   // Flag that indicates whether the task runner loop should continue. This is
   // only meant to be read/written on the thread executing RunUntilStopped().
@@ -141,7 +140,6 @@
 
   OSP_DISALLOW_COPY_AND_ASSIGN(TaskRunnerImpl);
 };
-}  // namespace platform
 }  // namespace openscreen
 
 #endif  // PLATFORM_IMPL_TASK_RUNNER_H_