GuestView: Decouple extension IPCs from soon-to-be component IPCs

In the feature, GuestViewMessageFilter will move to components and ExtensionsGuestViewMessageFilter will stay in the extensions module.

BUG=444869
TBR=rockot@chromium.org for non-guestview extensions, kenrb@chromium.org for _messages.h These are all just mechanical refactors.

Review URL: https://codereview.chromium.org/1092853002

Cr-Commit-Position: refs/heads/master@{#326082}


CrOS-Libchrome-Original-Commit: b0dc17d87cb785557687aea9532f60154acac198
diff --git a/ipc/ipc_message_start.h b/ipc/ipc_message_start.h
index 1cec310..3cf58f9 100644
--- a/ipc/ipc_message_start.h
+++ b/ipc/ipc_message_start.h
@@ -115,6 +115,7 @@
   NavigatorConnectMsgStart,
   CastMediaMsgStart,
   AwMessagePortMsgStart,
+  ExtensionsGuestViewMsgStart,
   GuestViewMsgStart,
   // Note: CastCryptoMsgStart and CastChannelMsgStart reserved for Chromecast
   // internal code. Contact gunsch@ before changing/removing.