Revert of [reland] Do not block in SimpleThread::Start(). (patchset #1 id:1 of https://codereview.chromium.org/2801073005/ )

Reason for revert:
Speculatively reverting hoping to resolve lots of random failures on 2 bots, starting with the builds below:

https://uberchromegw.corp.google.com/i/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%28dbg%29%281%29/builds/24574

https://uberchromegw.corp.google.com/i/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/40019

Original issue's description:
> [reland] Do not block in SimpleThread::Start().
>
> This CL is identical to https://codereview.chromium.org/2664953004.
> The race condition that it highlighted in a TaskScheduler test the
> first time that it landed was fixed by
> https://codereview.chromium.org/2798373002/
>
> There is no good reason to block in SimpleThread::Start().
> In addition to introducing unnecessary latency, blocking in
> Start() prevents a non-joinable DelegateSimpleThread from
> being deleted from Run() without external synchronization
> (it is wrong to delete SimpleThread while Start() is waiting
> on the |event_| member).
>
> BUG=
> [email protected],[email protected],[email protected]
> CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
>
> Review-Url: https://codereview.chromium.org/2801073005
> Cr-Commit-Position: refs/heads/master@{#462912}
> Committed: https://chromium.googlesource.com/chromium/src/+/d5acadb2d99dfbbde0cbe251173b566ceb5d3441

[email protected],[email protected],[email protected],[email protected]
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review-Url: https://codereview.chromium.org/2804393002
Cr-Commit-Position: refs/heads/master@{#462998}
10 files changed