IWYU fixes in /chrome/browser/extensions.

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=reillyg@chromium.org

Change-Id: I788d0ba88662c41a2bbe2f1feccd1558afe9cdf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2475034
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817562}

chrome/browser/extensions/external_registry_loader_win.cc

@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "base/bind.h"
+#include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_file.h"