Add SmbProviderClient weak getter

This change adds a SmbFileSystem::GetWeakSmbProviderClient() method for
getting a weak_ptr to the SmbProviderClient singleton. This weak_ptr will
be used for posting tasks in the form of base::OnceClosure to the SmbTaskQueue.

Bug: chromium:757625
Change-Id: I4e10bfd3b10a243cc65303a15e8f2e1249d1560b
Reviewed-on: https://chromium-review.googlesource.com/957307
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542952}

chrome/browser/chromeos/smb_client/smb_file_system.cc

@@ -148,6 +148,11 @@ SmbProviderClient* SmbFileSystem::GetSmbProviderClient() const {
   return chromeos::DBusThreadManager::Get()->GetSmbProviderClient();
 }
 
+base::WeakPtr<SmbProviderClient> SmbFileSystem::GetWeakSmbProviderClient()
+    const {
+  return GetSmbProviderClient()->AsWeakPtr();
+}
+
 void SmbFileSystem::Abort() {
   // TODO(zentaro): To implement Abort() fully will require storing a
   // request id unique to each method call and also passing it to the daemon.

chrome/browser/chromeos/smb_client/smb_file_system.h

@@ -228,6 +228,7 @@ class SmbFileSystem : public file_system_provider::ProvidedFileSystemInterface,
   int32_t GetMountId() const;
 
   SmbProviderClient* GetSmbProviderClient() const;
+  base::WeakPtr<SmbProviderClient> GetWeakSmbProviderClient() const;
 
   file_system_provider::ProvidedFileSystemInfo file_system_info_;
   file_system_provider::OpenedFiles opened_files_;