Demos.app: Add the fullscreen permission.

BUG=340534
TEST=none
TBR=xiyuan

Review URL: https://codereview.chromium.org/167123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251588 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251588