Revert of Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess.

Revert of Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess. (patchset #2 id:20001 of https://codereview.chromium.org/831373002/)

Reason for revert:
PrcessTest.CloneWithFlags fails on Linux ChromiumOS Ozone Tests (1):
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Ozone%20Tests%20%281%29/builds/8139/steps/base_unittests/logs/CloneFlags

ProcessTest.CloneFlags (run #1):
[ RUN      ] ProcessTest.CloneFlags
../../base/process/process_unittest.cc:241: Failure
Value of: process.IsValid()
  Actual: false
Expected: true
[  FAILED  ] ProcessTest.CloneFlags (0 ms)

Original issue's description:
> Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess.
>
> ForkWithFlags is a wrapper around the clone syscall that uses the libc
> clone wrapper, and thus updates the libc's pid cache if it has one
> (using sys_clone directly does not update the pid cache, so getpid may
> return an incorrect result in the child). This exposes the ability to
> set clone flags, which is needed to use Linux namespaces.
>
> BUG=312380
>
> Committed: https://crrev.com/e12d6652ece2a3ab72bb05837cfd7f0b0b9ecf3a
> Cr-Commit-Position: refs/heads/master@{#310327}

TBR=jln@chromium.org,mdempsky@chromium.org,thestig@chromium.org,mark@chromium.org,rickyz@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=312380
Committed: https://crrev.com/3e31085d2fe944c8fca2d2c06ca416469cc1942e
Cr-Commit-Position: refs/heads/master@{#310355}

[Marijn Kruisselbrink, 2015-01-07 20:30:05]

Created Revert of Move ForkWithFlags from sandbox/ to base/ and plug it into
LaunchProcess.

[commit-bot: I haz the power, 2015-01-07 20:32:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/842703002/1

[commit-bot: I haz the power, 2015-01-07 20:33:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-07 20:35:12]

Patchset 1 (id:??) landed as
https://crrev.com/3e31085d2fe944c8fca2d2c06ca416469cc1942e
Cr-Commit-Position: refs/heads/master@{#310355}