Revert "Remove primordial IPC Channel from GPU processes"

This reverts commit 6dc0acff58c55dc9821bb9cdcd4588c36f5c1586.

Reason for revert: Still breaks some GPU integration tests on Mac ASAN, probably timing issues.

Original change's description:
> Remove primordial IPC Channel from GPU processes
>
> GPU processes do not use this for anything other than monitoring process
> lifetime, implicitly through generic ChildProcessHostImpl and
> ChildThreadImpl behavior. Since these objects are now capable of
> coordinating process lifetime without a legacy IPC Channel present, the
> Channel does not need to exist at all for GPU processes. This CL removes
> it.
>
> Bug: 1196476
> Change-Id: Id4e5231303572f85c37048b58e3f7dbe791d4782
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2986000
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Commit-Queue: Ken Rockot <rockot@google.com>
> Cr-Commit-Position: refs/heads/master@{#895924}

Bug: 1196476
Change-Id: I36ba8bd735035b2c0a83e5731ed3dfdd76d9cb6d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2988012
Auto-Submit: Ken Rockot <rockot@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Owners-Override: Andy Paicu <andypaicu@google.com>
Cr-Commit-Position: refs/heads/master@{#896034}