Remove utility process batch mode support and its IPCs

The utility_process_host_impl batch mode feature has no users
anymore, nor is it a useful feature in the age of mojo. Ditch
the feature:

  - remove batch mode IPC and the message file.
  - remove StartBatchMode / EndBatchMode code.
  - change IPC unit tests to use AutomationMsgStart
    IPC start name (instead of UtilityMsgStart).
  - remove UtilityMsgStart IPC message start name.
  - utility_process_host_impl.*, document members
    and remove old / unused #include files.
  - utility_thread{,impl}.h, ditto.

No change in behavior, no new tests.

Bug: 738648
Change-Id: Ifcb65939ba7bf0a52451d2b8cb3a8a254399fcd2
Reviewed-on: https://chromium-review.googlesource.com/558526
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484557}

• content/browser/utility_process_host_impl.cc[diff]
• content/browser/utility_process_host_impl.h[diff]
• content/common/BUILD.gn[diff]
• content/common/content_message_generator.h[diff]
• content/common/utility_messages.h[Deleted - diff]
• content/public/browser/utility_process_host.h[diff]
• content/public/utility/utility_thread.h[diff]
• content/utility/utility_thread_impl.cc[diff]
• content/utility/utility_thread_impl.h[diff]
• ipc/OWNERS[diff]
• ipc/ipc_channel_proxy_unittest.cc[diff]
• ipc/ipc_channel_proxy_unittest_messages.h[diff]
• ipc/ipc_message_start.h[diff]