Make base:ReplaceFile return an informative error.

base/file_util_win.cc

Index: base/file_util_win.cc
diff --git a/base/file_util_win.cc b/base/file_util_win.cc
index 964302adce55b0cafab153460614e5bc2a81a1f3..66ed7283c1baca878466a263e248c4e80a8c68a0 100644
--- a/base/file_util_win.cc
+++ b/base/file_util_win.cc
@@ -148,8 +148,11 @@ bool MoveUnsafe(const FilePath& from_path, const FilePath& to_path) {
   return ret;
 }
 
-bool ReplaceFile(const FilePath& from_path, const FilePath& to_path) {
+bool ReplaceFile(const FilePath& from_path, const FilePath& to_path,
+                 base::PlatformFileError* error) {
   base::ThreadRestrictions::AssertIOAllowed();
+  if (error)
+    *error = base::PLATFORM_FILE_OK;
   // Try a simple move first.  It will only succeed when |to_path| doesn't
   // already exist.
   if (::MoveFile(from_path.value().c_str(), to_path.value().c_str()))
@@ -162,6 +165,8 @@ bool ReplaceFile(const FilePath& from_path, const FilePath& to_path) {
                     REPLACEFILE_IGNORE_MERGE_ERRORS, NULL, NULL)) {
     return true;
   }
+  if (error)
+    *error = base::LastErrorToPlatformFileError(GetLastError());
   return false;
 }