Update http_status_code_list.h comment.

This updates the comment to avoid the confusing include.

Change-Id: Ia573fa869ffedde49901f5ca5b31a819987cd544
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209775Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800935}

net/http/http_status_code_list.h

@@ -13,7 +13,7 @@
 // http://www.iana.org/assignments/http-status-codes/http-status-codes.xml
 
 #ifndef HTTP_STATUS
-#error "HTTP_STATUS should be defined before including this file"
+#error "Do #include net/http/http_status_code.h instead of this file directly."
 #endif
 
 // Informational 1xx