IWYU fixes in /chrome/updater.

base/win/registry.h includes more than it uses. These files were
inadvertently relying on that fact. This CL adds missing includes so
that registry.h can be cleaned up.

BUG=None
This CL was uploaded by git cl split.

R=ganesh@chromium.org

Change-Id: I9870f7a77b294b14475c63a4b2752de0d86ef87b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2475113
Commit-Queue: S. Ganesh <ganesh@chromium.org>
Reviewed-by: S. Ganesh <ganesh@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817492}

chrome/updater/win/group_policy_manager.h

@@ -8,6 +8,7 @@
 #include <memory>
 #include <string>
 
+#include "base/strings/string16.h"
 #include "base/win/registry.h"
 #include "chrome/updater/policy_manager.h"