Add a DCHECK to catch when a caller asks for no metadata.

Explority work to see if we can track down when an FSP is asked to get
metadata for a caller that doesn't want any metadata.

Bug: 587231
Change-Id: I5133016dd9bf01aeb66dc92010c8998d29067642
Reviewed-on: https://chromium-review.googlesource.com/1240553Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594549}

chrome/browser/chromeos/file_system_provider/operations/get_metadata.cc

@@ -142,7 +142,9 @@ GetMetadata::GetMetadata(
     : Operation(event_router, file_system_info),
       entry_path_(entry_path),
       fields_(fields),
-      callback_(std::move(callback)) {}
+      callback_(std::move(callback)) {
+  DCHECK_NE(0, fields_);
+}
 
 GetMetadata::~GetMetadata() {
 }