IWYU fix in startup_browser_policy_unittest.cc.

BUG=546640
R=tmartino@chromium.org


Change-Id: I0b1e57d40e2885e11c43d177a2a41e7354380eae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899449
Auto-Submit: Greg Thompson <grt@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712601}

chrome/browser/ui/startup/startup_browser_policy_unittest.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "base/files/file_util.h"
+#include "base/files/scoped_temp_dir.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/policy/browser_signin_policy_handler.h"
 #include "chrome/browser/prefs/browser_prefs.h"