Fix destruction order of FileSystemContext in MediaFileValidatorTest.

chrome/browser/media_galleries/fileapi/media_file_validator_browsertest.cc

 // Copyright 2013 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.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
@@ skipping 59 matching lines @@
 }
 
 }  // namespace
 
 class MediaFileValidatorTest : public InProcessBrowserTest {
  public:
   MediaFileValidatorTest() : test_file_size_(0) {}
 
   ~MediaFileValidatorTest() override {}
 
+  void TearDownOnMainThread() override {
+    // Release our ref to |file_system_context_| before the test framework winds
+    // down, otherwise releasing it in the destructor posts a destruction task
+    // to the FILE thread after it has been shutdown (which base/task_scheduler
+    // guards against in the RedirectNonUINonIOBrowserThreads experiment per the
+    // FILE thread's tasks being marked as shutdown blocking for legacy
+    // reasons).
+    file_system_context_ = nullptr;
+    InProcessBrowserTest::TearDownOnMainThread();
+  }
+
   // Write |content| into |filename| in a test file system and try to move
   // it into a media file system.  The result is compared to |expected_result|.
   void MoveTest(const std::string& filename, const std::string& content,
                 bool expected_result) {
     content::BrowserThread::PostTask(
         content::BrowserThread::FILE,
         FROM_HERE,
         base::Bind(&MediaFileValidatorTest::SetupOnFileThread,
                    base::Unretained(this), filename, content, expected_result));
     loop_runner_ = new content::MessageLoopRunner;
@@ skipping 200 matching lines @@
   test_file = test_file.AppendASCII("no_streams.webm");
   MoveTestFromFile("no_streams.webm", test_file, false);
 }
 
 IN_PROC_BROWSER_TEST_F(MediaFileValidatorTest, ValidVideo) {
   base::FilePath test_file = GetMediaTestDir();
   ASSERT_FALSE(test_file.empty());
   test_file = test_file.AppendASCII("bear-320x240-multitrack.webm");
   MoveTestFromFile("multitrack.webm", test_file, true);
 }