SmbService: Ensure DBusThreadManager is initialized before Get()

Prevents an issue once enabling SMB by default where the SmbService
ctor runs before the DBusThreadManager is properly initialized. This
only happens during tests.

Bug: chromium:757625
Change-Id: Id25265a7a9b763c5ea1ab59106df366dda4fd309
Reviewed-on: https://chromium-review.googlesource.com/1195644Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587298}

chrome/browser/chromeos/smb_client/smb_service.cc

@@ -199,6 +199,12 @@ Service* SmbService::GetProviderService() const {
 }
 
 SmbProviderClient* SmbService::GetSmbProviderClient() const {
+  // If the DBusThreadManager or the SmbProviderClient aren't available,
+  // there isn't much we can do. This should only happen when running tests.
+  if (!chromeos::DBusThreadManager::IsInitialized() ||
+      !chromeos::DBusThreadManager::Get()) {
+    return nullptr;
+  }
   return chromeos::DBusThreadManager::Get()->GetSmbProviderClient();
 }
 
@@ -266,6 +272,11 @@ void SmbService::StartSetup() {
     return;
   }
 
+  SmbProviderClient* client = GetSmbProviderClient();
+  if (!client) {
+    return;
+  }
+
   if (user->IsActiveDirectoryUser()) {
     auto account_id = user->GetAccountId();
     const std::string account_id_guid = account_id.GetObjGuid();