Linux sandbox: Make ChrootToSafeEmptyDir() faster.

Linux sandbox: Make ChrootToSafeEmptyDir() faster.

Use a vfork()-like system call instead of fork() in ChrootToSafeEmptyDir()
to avoid duplicating page tables, which is slow.

BUG=312380
Committed: https://crrev.com/1753adb1e43060ec322880f9a7bf9b483bac0ace
Cr-Commit-Position: refs/heads/master@{#312732}

[jln (very slow on Chromium), 2015-01-22 23:04:05]

jln@chromium.org changed reviewers:
+ rickyz@chromium.org

[jln (very slow on Chromium), 2015-01-22 23:04:11]

Ricky: PTAL!

[rickyz (no longer on Chrome), 2015-01-22 23:27:09]

lgtm

[jln (very slow on Chromium), 2015-01-22 23:34:20]

jln@chromium.org changed reviewers:
+ jorgelo@chromium.org

[jln (very slow on Chromium), 2015-01-22 23:34:28]

Jorge, would you mind stamping this?

[Jorge Lucangeli Obes, 2015-01-23 00:03:17]

lgtm

[jln (very slow on Chromium), 2015-01-23 00:09:35]

The CQ bit was checked by jln@chromium.org

[commit-bot: I haz the power, 2015-01-23 00:12:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/863933004/1

[commit-bot: I haz the power, 2015-01-23 01:26:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-23 01:26:58]

Patchset 1 (id:??) landed as
https://crrev.com/1753adb1e43060ec322880f9a7bf9b483bac0ace
Cr-Commit-Position: refs/heads/master@{#312732}