Reland of Enable the bootstrap sandbox by default.

Reland of Enable the bootstrap sandbox by default. (patchset #1 id:1 of https://codereview.chromium.org/1355043004/ )

Reason for revert:
Fixed issue that was the cause of the revert, https://crbug.com/534650.

Original issue's description:
> Revert of Enable the bootstrap sandbox by default. (patchset #3 id:40001 of https://codereview.chromium.org/1352623003/ )
>
> Reason for revert:
> Caused Mac 10.10 layout failures: http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.10/builds/4593
>
> Original issue's description:
> > Enable the bootstrap sandbox by default.
> >
> > This will place renderer processes under the bootstrap sandbox. This can be
> > disabled with --disable-bootstrap-sandbox.
> >
> > BUG=367863
> >
> > Committed: https://crrev.com/2dded5291bc5e9427721e1a06c74f2a80a86e8fa
> > Cr-Commit-Position: refs/heads/master@{#350006}
>
> TBR=avi@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=367863
>
> Committed: https://crrev.com/f1422520b42e8fa5b4ffe75359af96bac75d67b0
> Cr-Commit-Position: refs/heads/master@{#350065}

TBR=avi@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=367863
Committed: https://crrev.com/be226728db60e5860ab8d775eae36e406a41d7ee
Cr-Commit-Position: refs/heads/master@{#351386}

[Robert Sesek, 2015-09-29 18:31:07]

Created Reland of Enable the bootstrap sandbox by default.

[Robert Sesek, 2015-09-29 20:14:13]

The CQ bit was checked by rsesek@chromium.org

[commit-bot: I haz the power, 2015-09-29 20:14:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1372343004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1372343004/1

[commit-bot: I haz the power, 2015-09-29 20:16:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 20:17:06]

Patchset 1 (id:??) landed as
https://crrev.com/be226728db60e5860ab8d775eae36e406a41d7ee
Cr-Commit-Position: refs/heads/master@{#351386}

[Avi (use Gerrit), 2015-09-30 15:54:27]

lgtm

do it! do it!

[Robert Sesek, 2015-10-01 15:19:49]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1379133003/ by rsesek@chromium.org.

The reason for reverting is: Caused performance regressions:
https://crbug.com/537680.