[Mac] Shut down connections to WindowServer before engaging the sandbox.

This is only done for Mac OS versions 10.8 and higher.

A similar CL was landed and reverted:
https://chromium.googlesource.com/chromium/src.git/+/cef0cb9e60fb0f47562be83f8aa117899f0314d5

BUG=397642, 306348
TEST=Manually tested on 10.6-10.10.
R=avi@chromium.org, jeremy@chromium.org

Review URL: https://codereview.chromium.org/673443002

Cr-Commit-Position: refs/heads/master@{#300891}
(cherry picked from commit 4842a0e08daada491fc519b9eade2cfd04f804ed)

Review URL: https://codereview.chromium.org/698953002

Cr-Commit-Position: refs/branch-heads/2171@{#330}
Cr-Branched-From: 267aeeb8d85c8503a7fd12bd14654b8ea78d3974-refs/heads/master@{#297060}
1 file changed