Fix possible deadlock during shutdown by not using the FILE thread.

content/browser/browser_context.cc

Index: content/browser/browser_context.cc
diff --git a/content/browser/browser_context.cc b/content/browser/browser_context.cc
index 8755124590014af25d76024faf890a1d113a9c76..a81eb503a9db451a577b22ffb60e81183a96bf51 100644
--- a/content/browser/browser_context.cc
+++ b/content/browser/browser_context.cc
@@ -21,6 +21,7 @@
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/rand_util.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/browser/blob_storage/chrome_blob_storage_context.h"
@@ -479,9 +480,16 @@ void BrowserContext::Initialize(
     if (base::CommandLine::ForCurrentProcess()->HasSwitch(
             switches::kMojoLocalStorage)) {
       ServiceInfo info;
+      // TODO(mek): Use sequenced task runner rather than single thread task
+      // runner when mojo supports that.

[gab, 2017/05/03 20:18:57]

Add http://crbug.com/678155 for ease of searching.

But overall curious why this has to do with Mojo? Mojo isn't happy running its
services per se on sequences but it shouldn't care that the
"file_service_runner_" is a SequencedTaskRunner (just requires adjusting
FileService's interface to allow that? it doesn't appear to be truly
thread-affine, rather just thread-unsafe == sequence-affine?)

       info.factory =
           base::Bind(&file::CreateFileService,
-                     BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
+                     base::CreateSingleThreadTaskRunnerWithTraits(
+                         base::TaskTraits()
+                             .MayBlock()
+                             .WithBaseSyncPrimitives()
+                             .WithShutdownBehavior(
+                                 base::TaskShutdownBehavior::BLOCK_SHUTDOWN)),
                      BrowserThread::GetTaskRunnerForThread(BrowserThread::DB));
       connection->AddEmbeddedService(file::mojom::kServiceName, info);
     }