Remove MessageLoop references in /media/muxers

MessageLoop is going away soon.

This CL was uploaded by git cl split.

R=mcasas@chromium.org

Bug: 891670
Change-Id: Ib0e7dffcd366ce0a98793231e11386a83620828c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1945169
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720486}

media/muxers/webm_muxer_fuzzertest.cc

@@ -10,10 +10,10 @@
 
 #include "base/bind.h"
 #include "base/logging.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/stl_util.h"
 #include "base/strings/string_piece.h"
+#include "base/task/single_thread_task_executor.h"
 #include "media/base/audio_parameters.h"
 #include "media/base/video_frame.h"
 #include "media/muxers/webm_muxer.h"
@@ -37,7 +37,7 @@ static struct {
 struct Env {
   Env() { logging::SetMinLogLevel(logging::LOG_FATAL); }
 
-  base::MessageLoop message_loop;
+  base::SingleThreadTaskExecutor task_executor;
 };
 Env* env = new Env();