[IPC] Remove unused Chromecast *MsgStart messages

Part of the effort to eliminate the legacy IPC system.

Bug: 1007300
Change-Id: Idbe41f4be45af0be18eea4fd485bcf313dca480c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836494
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703659}

ipc/ipc_message_start.h

@@ -44,13 +44,8 @@ enum IPCMessageStart {
   OzoneGpuMsgStart,
   WebTestMsgStart,
   NetworkHintsMsgStart,
-  CastMediaMsgStart,
   ExtensionsGuestViewMsgStart,
   GuestViewMsgStart,
-  // Note: CastCryptoMsgStart and CastChannelMsgStart reserved for Chromecast
-  // internal code. Contact gunsch@ before changing/removing.
-  CastCryptoMsgStart,
-  CastChannelMsgStart,
   IPCTestMsgStart,
   MediaPlayerDelegateMsgStart,
   SurfaceViewManagerMsgStart,

tools/ipc_fuzzer/message_tools/message_list.cc

@@ -49,9 +49,6 @@ static bool check_msgtable() {
   exemptions.push_back(TestMsgStart);
   exemptions.push_back(BlinkTestMsgStart);
   exemptions.push_back(WebTestMsgStart);
-  exemptions.push_back(CastCryptoMsgStart);   // Reserved for chromecast.
-  exemptions.push_back(CastChannelMsgStart);  // Reserved for chromecast.
-  exemptions.push_back(CastMediaMsgStart);    // Reserved for chromecast.
   exemptions.push_back(IPCTestMsgStart);
   exemptions.push_back(WorkerMsgStart);    // Now only used by tests.
   exemptions.push_back(ChromeUtilityPrintingMsgStart);  // BUILDFLAGS, sigh.