aura: Fix unit conversion in test_screen

There's a couple of unit issues here that cause really strange behavior in
highdpi mode:

- The root window is created using scaled pixels, but CreateParams is
  actually defined to contain unscaled pixels. So the root window
  dimensions are always off by a factor of 2 in highdpi mode.

- The screen resizes the display using the wrong units. The screen bounds
  are in scaled pixels, but Display::SetSize takes pixels.

BUG=335713
TEST=ozone content_shell with --force-device-scale-factor=2

Review URL: https://codereview.chromium.org/142123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245933 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245933