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RenderThreadImpl::Shutdown has been shutting down Blink and V8 gracefully,
but the graceful shutdown has caused tons of use-after-free bugs
(and many engineers has spent lots of time fixing ordering issues around the shutdown).

As discussed in chromium-dev@ (https://groups.google.com/a/chromium.org/d/topic/chromium-dev/OLS4JSZvowI/discussion) and
platform-architecture-dev@ (https://groups.google.com/a/chromium.org/d/topic/platform-architecture-dev/Zc12k91NTFk/discussion),
there is no reason we have to shut down the renderer gracefully.
It's just causing use-after-free bugs and wasting performance.

Hence this CL removes RenderThreadImpl::Shutdown and instead calls _exit(0) in ~ChildProcess.
We need a couple of special handling in a single-process mode. See the comment in ~ChildProcess
for more details.

BUG=639244

Review-Url: https://codereview.chromium.org/2309153002
Cr-Commit-Position: refs/heads/master@{#443175}