Move GuestView IPCs into their own messages file.

BUG=444869

Review URL: https://codereview.chromium.org/892923002

Cr-Commit-Position: refs/heads/master@{#314233}


CrOS-Libchrome-Original-Commit: a04a4483105ca6f5daadc992536e839ee2f6c18f
diff --git a/ipc/ipc_message_start.h b/ipc/ipc_message_start.h
index 48dd9af..5305d22 100644
--- a/ipc/ipc_message_start.h
+++ b/ipc/ipc_message_start.h
@@ -114,6 +114,7 @@
   NavigatorConnectMsgStart,
   CastMediaMsgStart,
   AwMessagePortMsgStart,
+  GuestViewMsgStart,
   LastIPCMsgStart  // Must come last.
 };