Add missing includes of //base/dcheck_is_on.h in //base

Bug: 1300573
Change-Id: Iab9ab635ccedda23a5ed8e017496918045d962b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3587350
Commit-Queue: David Sanders <dsanders11@ucsbalum.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Owners-Override: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#992789}

diff --git a/base/observer_list.h b/base/observer_list.h
index 1822ab5..3270e31 100644
--- a/base/observer_list.h
+++ b/base/observer_list.h

@@ -17,6 +17,7 @@
 
 #include "base/check.h"
 #include "base/check_op.h"
+#include "base/dcheck_is_on.h"
 #include "base/notreached.h"
 #include "base/observer_list_internal.h"
 #include "base/ranges/algorithm.h"