Revert of Move content's shell connections to the IO thread

content/browser/mojo/mojo_shell_context.cc

Index: content/browser/mojo/mojo_shell_context.cc
diff --git a/content/browser/mojo/mojo_shell_context.cc b/content/browser/mojo/mojo_shell_context.cc
index 37b9e7011651132e7af28ddc1e70f69754d23e21..24e21ca8471d9e497dd5214a45a028d521becb2f 100644
--- a/content/browser/mojo/mojo_shell_context.cc
+++ b/content/browser/mojo/mojo_shell_context.cc
@@ -49,10 +49,20 @@
 
 namespace {
 
-base::LazyInstance<std::unique_ptr<shell::Connector>>::Leaky
-    g_io_thread_connector = LAZY_INSTANCE_INITIALIZER;
-
-void DestroyConnectorOnIOThread() { g_io_thread_connector.Get().reset(); }
+using ConnectorPtr = base::ThreadLocalPointer<shell::Connector>;
+
+base::LazyInstance<ConnectorPtr>::Leaky io_connector_tls_ptr =
+    LAZY_INSTANCE_INITIALIZER;
+
+void SetConnectorOnIOThread(std::unique_ptr<shell::Connector> connector) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
+  io_connector_tls_ptr.Pointer()->Set(connector.release());
+}
+
+void DestroyConnectorOnIOThread() {
+  delete MojoShellContext::GetConnectorForIOThread();
+  io_connector_tls_ptr.Pointer()->Set(nullptr);
+}
 
 void StartUtilityProcessOnIOThread(
     mojo::InterfaceRequest<mojom::ProcessControl> request,
@@ -262,9 +272,14 @@
     request =
         service_manager_->StartEmbedderService(kBrowserMojoApplicationName);
   }
-  MojoShellConnection::SetForProcess(MojoShellConnection::Create(
-      std::move(request),
-      BrowserThread::GetTaskRunnerForThread(BrowserThread::IO)));
+  MojoShellConnection::SetForProcess(
+      MojoShellConnection::Create(std::move(request)));
+
+  std::unique_ptr<shell::Connector> io_connector =
+      MojoShellConnection::GetForProcess()->GetConnector()->Clone();
+  BrowserThread::PostTask(
+      BrowserThread::IO, FROM_HERE,
+      base::Bind(&SetConnectorOnIOThread, base::Passed(&io_connector)));
 
   ContentBrowserClient::StaticMojoApplicationMap apps;
   GetContentClient()->browser()->RegisterInProcessMojoApplications(&apps);
@@ -272,13 +287,6 @@
     MojoShellConnection::GetForProcess()->AddEmbeddedService(entry.first,
                                                              entry.second);
   }
-
-  // This is safe to assign directly from any thread, because MojoShellContext
-  // must be constructed before anyone can call GetConnectorForIOThread().
-  g_io_thread_connector.Get() =
-      MojoShellConnection::GetForProcess()->GetConnector()->Clone();
-
-  MojoShellConnection::GetForProcess()->Start();
 
   ContentBrowserClient::OutOfProcessMojoApplicationMap sandboxed_apps;
   GetContentClient()
@@ -332,7 +340,7 @@
 // static
 shell::Connector* MojoShellContext::GetConnectorForIOThread() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
-  return g_io_thread_connector.Get().get();
+  return io_connector_tls_ptr.Pointer()->Get();
 }
 
 void MojoShellContext::ConnectToApplicationOnOwnThread(