base: Move MemoryMappedFile out of file_util.h and into its own header file.

base/test/test_file_util_mac.cc

Index: base/test/test_file_util_mac.cc
diff --git a/base/test/test_file_util_mac.cc b/base/test/test_file_util_mac.cc
index b52b39c337a1ad5c4a7bf15efa6d80311668f6eb..3af2c105d9cbce0bde1d63aa16a8ce105fe23698 100644
--- a/base/test/test_file_util_mac.cc
+++ b/base/test/test_file_util_mac.cc
@@ -6,8 +6,10 @@
 
 #include <sys/mman.h>
 #include <errno.h>
+
 #include "base/logging.h"
 #include "base/file_util.h"
+#include "base/files/memory_mapped_file.h"
 
 namespace file_util {
 
@@ -30,7 +32,7 @@ bool EvictFileFromSystemCache(const base::FilePath& file) {
     return true;
   }
 
-  file_util::MemoryMappedFile mapped_file;
+  base::MemoryMappedFile mapped_file;
   if (!mapped_file.Initialize(file)) {
     DLOG(WARNING) << "failed to memory map " << file.value();
     return false;