Add calls to WrapCurrentThread and UnwrapCurrentThread.

Needed after the move of the rtc::Task class and its tests, because
webrtc in chrome is built with NO_MAIN_THREAD_WRAPPING.

BUG=708968

Review-Url: https://codereview.chromium.org/2803023002
Cr-Commit-Position: refs/heads/master@{#462802}

third_party/libjingle_xmpp/task_runner/task_unittest.cc

@@ -97,6 +97,9 @@ class HappyTask : public IdTimeoutTask {
 
 class MyTaskRunner : public TaskRunner {
  public:
+  MyTaskRunner() { ThreadManager::Instance()->WrapCurrentThread(); }
+  ~MyTaskRunner() { ThreadManager::Instance()->UnwrapCurrentThread(); }
+
   virtual void WakeTasks() { RunTasks(); }
   virtual int64_t CurrentTime() { return GetCurrentTime(); }