Explicitly std::move() returned std::vector<Mapping>.

Otherwise, C++11-compliant compilers may select PosixErrorOr(const T&)
as the relevant constructor, and fail because std::vector<Mapping> is
not copyable (because Mapping is not copyable).

This is a C++11 defect that is fixed in C++14 (and in C++11 mode for
Clang, but not GCC). See DR 1579,
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579.

PiperOrigin-RevId: 230767401
Change-Id: I65f481f5188d91db6cbbbd65ed0a60bc55df3401

test/syscalls/linux/socket_unix_non_stream.cc

@@ -106,7 +106,7 @@ PosixErrorOr<std::vector<Mapping>> CreateFragmentedRegion(const int size,
   // The mappings above have taken ownership of the region.
   region.release();
 
-  return pages;
+  return std::move(pages);
 }
 
 // A contiguous iov that is heavily fragmented in FileMem can still be sent