Remove BrowserPluginMsgStart and NetworkHintsMsgStart from enum IPCMessageStart

They have become unused.

Change-Id: I1090f20a31c9f72dc494fff07b08ff49d206d8ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989868
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729516}

ipc/ipc_message_start.h

@@ -33,7 +33,6 @@ enum IPCMessageStart {
   AccessibilityMsgStart,
   PrerenderMsgStart,
   ChromotingMsgStart,
-  BrowserPluginMsgStart,
   AndroidWebViewMsgStart,
   NaClHostMsgStart,
   EncryptedMediaMsgStart,
@@ -42,7 +41,6 @@ enum IPCMessageStart {
   ChromeUtilityPrintingMsgStart,
   OzoneGpuMsgStart,
   WebTestMsgStart,
-  NetworkHintsMsgStart,
   ExtensionsGuestViewMsgStart,
   GuestViewMsgStart,
   MediaPlayerDelegateMsgStart,