[IPC] Remove unused *MsgStart values

Part of the elimination of the legacy IPC system.

Bug: 1007300
Change-Id: I3c8f3e0e17c157be07b494217e8cbe87edbb990f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832802
Auto-Submit: Colin Blundell <blundell@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701957}

ipc/ipc_message_start.h

@@ -24,10 +24,7 @@ enum IPCMessageStart {
   GpuChannelMsgStart,
   MediaMsgStart,
   PpapiMsgStart,
-  DOMStorageMsgStart,
   ResourceMsgStart,
-  BlobMsgStart,
-  MidiMsgStart,
   ChromeMsgStart,
   DragMsgStart,
   PrintMsgStart,
@@ -40,10 +37,8 @@ enum IPCMessageStart {
   BrowserPluginMsgStart,
   AndroidWebViewMsgStart,
   MediaPlayerMsgStart,
-  TracingMsgStart,
   PeerConnectionTrackerMsgStart,
   WebRtcLoggingMsgStart,
-  TtsMsgStart,
   NaClHostMsgStart,
   EncryptedMediaMsgStart,
   CastMsgStart,