[CrOS MultiDevice] Add flag for Messages usage of messages.google.com.

This flag protects the migration described at go/awm-cros-domain.

Bug: 917855
Change-Id: If2519bba5ee8c07af4bb1410942ba0f12ff66b4b
Reviewed-on: https://chromium-review.googlesource.com/c/1391311
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Jeremy Klein <[email protected]>
Commit-Queue: Kyle Horimoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#618949}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index b172b33..4dbe4ec 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -60,6 +60,9 @@
 extern const char kAndroidMessagesIntegrationName[];
 extern const char kAndroidMessagesIntegrationDescription[];
 
+extern const char kUseMessagesGoogleComDomainName[];
+extern const char kUseMessagesGoogleComDomainDescription[];
+
 extern const char kAndroidMessagesProdEndpointName[];
 extern const char kAndroidMessagesProdEndpointDescription[];