Remove forward declarations of MessageLoopImpl

MessageLoopImpl was removed by crrev.com/c/1440828, but there are still
somce forward declarations around.

Change-Id: I39d1e18632b97aa6a7aa2e59fca61db997651034
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974817
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726676}

base/message_loop/message_loop.h

@@ -23,7 +23,6 @@
 
 namespace base {
 
-class MessageLoopImpl;
 class MessagePump;
 class TaskObserver;
 

base/message_loop/message_loop_current.h

@@ -23,10 +23,7 @@ class WebTaskEnvironment;
 
 namespace base {
 
-class MessageLoopImpl;
-
 namespace sequence_manager {
-
 namespace internal {
 class SequenceManagerImpl;
 }
@@ -181,7 +178,6 @@ class BASE_EXPORT MessageLoopCurrent {
   static sequence_manager::internal::SequenceManagerImpl*
   GetCurrentSequenceManagerImpl();
 
-  friend class MessageLoopImpl;
   friend class MessagePumpLibeventTest;
   friend class ScheduleWorkTest;
   friend class Thread;

base/threading/thread_restrictions.h

@@ -299,7 +299,6 @@ class TaskTracker;
 class AdjustOOMScoreHelper;
 class FileDescriptorWatcher;
 class GetAppOutputScopedAllowBaseSyncPrimitives;
-class MessageLoopImpl;
 class ScopedAllowThreadRecallForStackSamplingProfiler;
 class SimpleThread;
 class StackSamplingProfiler;
@@ -475,7 +474,6 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitivesOutsideBlockingScope {
   friend class base::sequence_manager::internal::TaskQueueImpl;
   friend class base::FileDescriptorWatcher;
   friend class base::internal::JobTaskSource;
-  friend class base::MessageLoopImpl;
   friend class base::ScopedAllowThreadRecallForStackSamplingProfiler;
   friend class base::StackSamplingProfiler;
   friend class blink::RTCVideoDecoderAdapter;