Replace forward declarations with includes in TaskRunner APIs.

This CL:
- Adds #include "base/location.h" in base/task_runner.h
- Adds #include "base/single_thread_task_runner.h"
      in base/threading/thread_task_runner_handle.h
- Adds #include "base/sequenced_task_runner.h"
      in base/threading/sequenced_task_runner_handle.h

This will be followed by a mass removal of includes that
are no longer necessary.

BUG=618793

Review-Url: https://codereview.chromium.org/2053503004
Cr-Commit-Position: refs/heads/master@{#399044}

base/task_runner.h

@@ -9,13 +9,10 @@
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
+#include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 
-namespace tracked_objects {
-class Location;
-} // namespace tracked_objects
-
 namespace base {
 
 struct TaskRunnerTraits;

base/threading/sequenced_task_runner_handle.cc

@@ -8,7 +8,6 @@
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
-#include "base/sequenced_task_runner.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_local.h"
 #include "base/threading/thread_task_runner_handle.h"

base/threading/sequenced_task_runner_handle.h

@@ -8,11 +8,10 @@
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/sequenced_task_runner.h"
 
 namespace base {
 
-class SequencedTaskRunner;
-
 class BASE_EXPORT SequencedTaskRunnerHandle {
  public:
   // Returns a SequencedTaskRunner which guarantees that posted tasks will only

base/threading/thread_task_runner_handle.cc

@@ -8,7 +8,6 @@
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
-#include "base/single_thread_task_runner.h"
 #include "base/threading/sequenced_task_runner_handle.h"
 #include "base/threading/thread_local.h"
 

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().