shell_aura: Set child to root window size, not host size

The host size is in pixels and the root window size is in scaled pixels.
So, using the pixel size may make the child window much larger than the
root window (and screen). Fix this by matching the root window size.

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

Review URL: https://codereview.chromium.org/141853003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246389 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246389