IWYU: size_t is defined stddef.h

Change-Id: I4400ac7c6004b49ec6e72c44f2754e2166642f88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552586Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830010}

components/bookmarks/browser/bookmark_model_observer.h

@@ -7,6 +7,8 @@
 
 #include <set>
 
+#include <stddef.h>
+
 class GURL;
 
 namespace bookmarks {