Make base:ReplaceFile return an informative error.

base/file_util_posix.cc

Index: base/file_util_posix.cc
diff --git a/base/file_util_posix.cc b/base/file_util_posix.cc
index 8b368127410e091eb392cf440f2bd9c50a417acd..b3bb27d1e8a6a93aa454a122a93939757a6b818d 100644
--- a/base/file_util_posix.cc
+++ b/base/file_util_posix.cc
@@ -232,9 +232,14 @@ bool MoveUnsafe(const FilePath& from_path, const FilePath& to_path) {
   return true;
 }
 
-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();
-  return (rename(from_path.value().c_str(), to_path.value().c_str()) == 0);
+  if (rename(from_path.value().c_str(), to_path.value().c_str()) == 0)
+    return true;
+  if (error)
+    *error = base::ErrnoToPlatformFileError(errno);
+  return false;
 }
 
 bool CopyDirectory(const FilePath& from_path,