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/596070
Reviewed-by: John Abd-El-Malek <[email protected]>
Commit-Queue: Tom Sepez <[email protected]>
Cr-Commit-Position: refs/heads/master@{#491404}
1 file changed