Chromecast: adds CastCryptoMsgStart to IPCMessageStart list.

R=tsepez@chromium.org
BUG=336640

Review URL: https://codereview.chromium.org/942683003

Cr-Commit-Position: refs/heads/master@{#317710}

ipc/ipc_message_start.h

@@ -115,6 +115,9 @@ enum IPCMessageStart {
   CastMediaMsgStart,
   AwMessagePortMsgStart,
   GuestViewMsgStart,
+  // Note: CastCryptoMsgStart reserved for Chromecast internal code.
+  // Contact gunsch@ before changing/removing.
+  CastCryptoMsgStart,
   LastIPCMsgStart  // Must come last.
 };