Revert of Set a new TLS when using CLONE_VM.

Revert of Set a new TLS when using CLONE_VM. (patchset #4 id:80001 of https://codereview.chromium.org/1248673004/)

Reason for revert:
This CL has broken ASan Linux (sandboxed) tests: basically every test from browser_tests, content_browsertests, interactive_ui_tests times out: http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20Tests%20%28sandboxed%29/builds/15502

Original issue's description:
> Set a new TLS when using CLONE_VM.
>
> On some libcs, clone writes to the new child's TLS before returning, so
> we set a new TLS to avoid corrupting the parent process's TLS.
>
> On glibc, this caused an assertion failure inside of
> pthread_getattr_np(pthread_self(), &attr).
>
> BUG=512623
>
> Committed: https://crrev.com/d2516cf1c3843541c920a476315e2a6f10cb0f92
> Cr-Commit-Position: refs/heads/master@{#340025}

TBR=jln@chromium.org,rickyz@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=512623
Committed: https://crrev.com/eccffe499e5a28eeaa68a135008ddd7a8534bdf7
Cr-Commit-Position: refs/heads/master@{#340069}

[Alexander Potapenko, 2015-07-23 11:01:57]

Created Revert of Set a new TLS when using CLONE_VM.

[commit-bot: I haz the power, 2015-07-23 11:02:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1256533002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1256533002/1

[commit-bot: I haz the power, 2015-07-23 11:03:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-23 11:04:06]

Patchset 1 (id:??) landed as
https://crrev.com/eccffe499e5a28eeaa68a135008ddd7a8534bdf7
Cr-Commit-Position: refs/heads/master@{#340069}