ozone: Use gfx::Display size for touch scaling

The old default causes weird scaling on the Chromebook Pixel. Try to use
the display size to set the scale.

TEST=content_shell with embedded=1 on pixel
TBR=rjkroege

Review URL: https://codereview.chromium.org/131373007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251075 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251075