gesture-nav: Introduce a 'simple' option for the gesture-nav UI.

BUG=344573
R=sky@chromium.org

Review URL: https://codereview.chromium.org/171133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251847 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251847