[message_loop] Remove message_loop_forward.h

As a final step, replace all includes back:
- mv message_loop_current.h message_loop.h
- s/message_loop_forward.h/message_loop.h/ in all includes.
- s/message_loop_forward.h/message_loop.h/ in base/BUILD.gn
- Remove message_loop_forward.h from third_party/DEPS.

[email protected]
[email protected]
BUG=891670

Change-Id: I623077025701459ddb7045cbcfdad138aa90a9e4
Reviewed-on: https://chromium-review.googlesource.com/c/1313110
Reviewed-by: Alexander Timin <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#604690}
diff --git a/media/base/null_video_sink_unittest.cc b/media/base/null_video_sink_unittest.cc
index 7996822..2b4c5e1 100644
--- a/media/base/null_video_sink_unittest.cc
+++ b/media/base/null_video_sink_unittest.cc
@@ -7,7 +7,7 @@
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/macros.h"
-#include "base/message_loop/message_loop_forward.h"
+#include "base/message_loop/message_loop.h"
 #include "base/test/simple_test_tick_clock.h"
 #include "media/base/gmock_callback_support.h"
 #include "media/base/null_video_sink.h"