Enable pinch-to-zoom on Windows 8 by default

This is the android style PtZ. It has known issues wrt to fixed-position
elements.

This is effective in desktop and metro experiences.

BUG=338498

Review URL: https://codereview.chromium.org/148083009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247390 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247390