[ImportantFileWriter] Improve diagnostics on failure to replace.

On Windows, ImportantFileWriter boosts its thread's priority for the
moment it calls ::ReplaceFileW in the hopes that this will improve its
chances of beating other processes (e.g., A/V scanners) that will try to
open the files it needs. This change preserves the last error code
coming out of a failure of ::ReplaceFileW for use in logging and metrics
reporting.

BUG=1145235
R=jdoerrie@chromium.org

Change-Id: I1e275698c5c39fa7cfa97d6b3bf48e2023a8763f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556841
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830557}

base/files/important_file_writer.cc

@@ -274,8 +274,13 @@ bool ImportantFileWriter::WriteFileAtomicallyImpl(const FilePath& path,
   tmp_file.Close();
   const bool result = ReplaceFile(tmp_file_path, path, &replace_file_error);
 #if defined(OS_WIN)
+  // Save and restore the last error code so that it's not polluted by the
+  // thread priority change.
+  const auto last_error = ::GetLastError();
   if (reset_priority)
     PlatformThread::SetCurrentThreadPriority(previous_priority);
+  if (!result)
+    ::SetLastError(last_error);
 #endif  // defined(OS_WIN)
 
   if (!result) {