Make AppShimHostManager a RefCountedThreadSafe.

apps/app_shim/app_shim_host_manager_mac.mm

Index: apps/app_shim/app_shim_host_manager_mac.mm
diff --git a/apps/app_shim/app_shim_host_manager_mac.mm b/apps/app_shim/app_shim_host_manager_mac.mm
index 049ba5b4fea07c95f2aa1be57c92da73f51bd18c..c364c1a6a05a366da293be71e9b1416f106d5908 100644
--- a/apps/app_shim/app_shim_host_manager_mac.mm
+++ b/apps/app_shim/app_shim_host_manager_mac.mm
@@ -31,8 +31,7 @@ AppShimHostManager::AppShimHostManager() {
   apps::AppShimHandler::SetDefaultHandler(&extension_app_shim_handler_);
   BrowserThread::PostTask(
       BrowserThread::FILE, FROM_HERE,
-      base::Bind(&AppShimHostManager::InitOnFileThread,
-          base::Unretained(this)));
+      base::Bind(&AppShimHostManager::InitOnFileThread, this));
 }
 
 AppShimHostManager::~AppShimHostManager() {
@@ -52,8 +51,7 @@ void AppShimHostManager::InitOnFileThread() {
   factory_.reset(new IPC::ChannelFactory(socket_path, this));
   BrowserThread::PostTask(
       BrowserThread::IO, FROM_HERE,
-      base::Bind(&AppShimHostManager::ListenOnIOThread,
-          base::Unretained(this)));
+      base::Bind(&AppShimHostManager::ListenOnIOThread, this));
 }
 
 void AppShimHostManager::ListenOnIOThread() {