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 509baef956e1a3f77358aace528bdfe20057ac38..a834183d8b18007c6109feb5d4c41658cc2d663e 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"
@@ -480,9 +481,16 @@ 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 (http://crbug.com/678155).
       info.factory =
           base::Bind(&file::CreateFileService,
-                     BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
+                     base::CreateSingleThreadTaskRunnerWithTraits(
+                         base::TaskTraits()

[gab, 2017/05/08 17:14:32]

See brand new way to specify TaskTraits as a bag instead of builder pattern
(constexpr and much less verbose :)):
https://cs.chromium.org/chromium/src/base/task_scheduler/task_traits.h?rcl=58...

[Marijn Kruisselbrink, 2017/05/08 18:18:01]

Ah, nice. Done. The sample code in post_task.h should probably be updated to the
new pattern too?

[gab, 2017/05/08 19:25:14]

Ah oops looks like we forgot to update that, will do, thanks!

+                             .MayBlock()
+                             .WithBaseSyncPrimitives()

[gab, 2017/05/08 17:14:32]

Do you really need WithBaseSyncPrimitives()? (see its API)

[Marijn Kruisselbrink, 2017/05/08 18:18:01]

Ah yes, it indeed seems that to signal on a waitable event you don't need
WithBaseSyncPrimitives, only to wait for it it's needed? Removed it. The other
task runner here (which currently still uses the DB thread), as well as a third
thread internal in the leveldb mojo code do need WithBaseSyncPrimitives as they
actually wait on waitable events, but this indeed doesn't.

+                             .WithShutdownBehavior(
+                                 base::TaskShutdownBehavior::BLOCK_SHUTDOWN)),
                      BrowserThread::GetTaskRunnerForThread(BrowserThread::DB));
       connection->AddEmbeddedService(file::mojom::kServiceName, info);
     }