Return success in WriteFile when writing from an empty buffer.

Changes WriteFile to return without calling D-Bus when attempting to write from an empty buffer.

Bug: chromium:757625
Change-Id: I386a571347ccf84785147ae91edea515b84a507a
Reviewed-on: https://chromium-review.googlesource.com/1005776Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551047}

chrome/browser/chromeos/smb_client/smb_file_system.cc

@@ -387,6 +387,11 @@ AbortCallback SmbFileSystem::WriteFile(
     int64_t offset,
     int length,
     storage::AsyncFileUtil::StatusCallback callback) {
+  if (length == 0) {
+    std::move(callback).Run(base::File::FILE_OK);
+    return CreateAbortCallback();
+  }
+
   const std::vector<uint8_t> data(buffer->data(), buffer->data() + length);
   base::ScopedFD temp_fd = temp_file_manager_.CreateTempFile(data);