Replace forward declarations with includes in TaskRunner APIs.

base/threading/thread_task_runner_handle.h

Index: base/threading/thread_task_runner_handle.h
diff --git a/base/threading/thread_task_runner_handle.h b/base/threading/thread_task_runner_handle.h
index 72ce49e1bd045bf2f7603ef8afc5844936dffc06..c8e58935f0977259349857bf91d71c63bf734567 100644
--- a/base/threading/thread_task_runner_handle.h
+++ b/base/threading/thread_task_runner_handle.h
@@ -8,11 +8,10 @@
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 
 namespace base {
 
-class SingleThreadTaskRunner;
-
 // ThreadTaskRunnerHandle stores a reference to a thread's TaskRunner
 // in thread-local storage.  Callers can then retrieve the TaskRunner
 // for the current thread by calling ThreadTaskRunnerHandle::Get().