Replace FFmpegDemuxer thread per element with base::TaskScheduler.

media/test/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//media/media_options.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 
 source_set("run_all_unittests") {
   testonly = true
   sources = [
@@ skipping 21 matching lines @@
       "pipeline_integration_test_base.cc",
       "pipeline_integration_test_base.h",
     ]
 
     public_deps = [
       "//media",
     ]
 
     deps = [
       "//base",
+      "//base/test:test_support",
       "//media:test_support",
       "//media/audio:test_support",
       "//media/base:test_support",
       "//testing/gmock",
       "//testing/gtest",
     ]
   }
 }
 
 source_set("pipeline_integration_tests") {
@@ skipping 103 matching lines @@
   ]
 
   libfuzzer_options = [
     # This is done to avoid DEATH of ASan with "Thread limit exceeded" error.
     "runs=500000",
 
     # This is done to suppress tons of log messages generated by gmock asserts.
     "close_fd_mask=1",
   ]
 }