Cleanup return values in SnapshotFileDetails

This CL turns const by value return values into const by ref.

Replacing const value by a const ref when a member variable is safe, because
the member variable will live at least as long as the temporary object returned
now. It can also be more efficient, because returning a ref can spare copying.

BUG=393155

Review URL: https://codereview.chromium.org/646853003

Cr-Commit-Position: refs/heads/master@{#299161}

chrome/browser/media_galleries/linux/snapshot_file_details.h

@@ -59,7 +59,7 @@ class SnapshotFileDetails {
     return bytes_written_;
   }
 
-  const base::File::Info file_info() const {
+  const base::File::Info& file_info() const {
     return file_info_;
   }