[Xenial64]Browser page no "Leave private mode" selection on the menu.

[Hannah Zhu]

Information:
phablet@ubuntu-phablet:~$ system-image-cli -i
current build number: 78
device name: frieza_arm64
channel: ubuntu-touch/staging/ubuntu
last update: 2016-11-09 07:13:12
version version: 78
version ubuntu: 20161109
version device: 20161014.0
version custom: 20161109

Step:
1.Open Browser app
2.Press the menu on the top right corner
3.Select "New private window"
4.On the private window page, press the menu again

Actual result:
After step 4, there no "Leave private window" selection.

Expect result:
Should be a leave private window selection.