Remove unused constants from ipc_message_start.h (try #2).

Split off from https://chromium-review.googlesource.com/c/584857
so that the other non-mechanical cleanup work wouldn't be lost if
this gets reverted.  See that CL for signoff.

Change-Id: Ic75adc756990d3182aba8a323021f328b8419754
Reviewed-on: https://chromium-review.googlesource.com/596070Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491404}

ipc/ipc_message_start.h

@@ -8,13 +8,15 @@
 // Used by IPC_BEGIN_MESSAGES so that each message class starts from a unique
 // base.  Messages have unique IDs across channels in order for the IPC logging
 // code to figure out the message class from its ID.
+//
+// You should no longer be adding any new message classes. Instead, use mojo
+// for all new work.
 enum IPCMessageStart {
   AutomationMsgStart = 0,
   FrameMsgStart,
   PageMsgStart,
   ViewMsgStart,
   InputMsgStart,
-  ProfileImportMsgStart,
   TestMsgStart,
   DevToolsMsgStart,
   WorkerMsgStart,
@@ -43,17 +45,12 @@ enum IPCMessageStart {
   PrintMsgStart,
   SpellCheckMsgStart,
   ExtensionMsgStart,
-  VideoCaptureMsgStart,
   QuotaMsgStart,
   TextInputClientMsgStart,
-  ChromeUtilityMsgStart,
   MediaStreamMsgStart,
-  ChromeBenchmarkingMsgStart,
   JavaBridgeMsgStart,
-  GamepadMsgStart,
   ShellMsgStart,
   AccessibilityMsgStart,
-  PrefetchMsgStart,
   PrerenderMsgStart,
   ChromotingMsgStart,
   BrowserPluginMsgStart,
@@ -63,37 +60,27 @@ enum IPCMessageStart {
   MediaPlayerMsgStart,
   TracingMsgStart,
   PeerConnectionTrackerMsgStart,
-  VisitedLinkMsgStart,
   AppShimMsgStart,
   WebRtcLoggingMsgStart,
   TtsMsgStart,
-  WebSocketMsgStart,
   NaClHostMsgStart,
-  WebRTCIdentityMsgStart,
-  PowerMonitorMsgStart,
   EncryptedMediaMsgStart,
   CacheStorageMsgStart,
   ServiceWorkerMsgStart,
-  MessagePortMsgStart,
   EmbeddedWorkerMsgStart,
   EmbeddedWorkerContextMsgStart,
   CastMsgStart,
-  CdmMsgStart,
   MediaStreamTrackMetricsHostMsgStart,
   ChromeExtensionMsgStart,
-  PushMessagingMsgStart,
   GinJavaBridgeMsgStart,
   ChromeUtilityPrintingMsgStart,
   AecDumpMsgStart,
   OzoneGpuMsgStart,
   ChromeUtilityExtensionsMsgStart,
   PlatformNotificationMsgStart,
-  PDFMsgStart,
   LayoutTestMsgStart,
   NetworkHintsMsgStart,
-  BluetoothMsgStart,
   CastMediaMsgStart,
-  AwMessagePortMsgStart,
   SyncCompositorMsgStart,
   ExtensionsGuestViewMsgStart,
   GuestViewMsgStart,
@@ -101,16 +88,8 @@ enum IPCMessageStart {
   // internal code. Contact gunsch@ before changing/removing.
   CastCryptoMsgStart,
   CastChannelMsgStart,
-  DataReductionProxyStart,
-  ChromeAppBannerMsgStart,
-  AttachmentBrokerMsgStart,
   RenderProcessMsgStart,
-  PageLoadMetricsMsgStart,
   IPCTestMsgStart,
-  ArcInstanceMsgStart,
-  ArcInstanceHostMsgStart,
-  DistillerMsgStart,
-  ArcCameraMsgStart,
   DWriteFontProxyMsgStart,
   MediaPlayerDelegateMsgStart,
   SurfaceViewManagerMsgStart,