linux_aura: Enable HIDPI support

Do the preparation work for enabling HIDPI support on linux aura: enable DIP and calculate device scale factor at DesktopScreenX11.

Review URL: https://codereview.chromium.org/27156003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235967 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235967