add perspective division to computing bounding box
add perspective division to computing bounding box

BUG=chromium-os:5614
TEST=ran overview mode and drag windows

Review URL: http://codereview.chromium.org/3124014

joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager