wm: enable compositing optimization by default
wm: enable compositing optimization by default

BUG=chromium-os:1103
TEST=tested all scenarios that were causing problems

Review URL: http://codereview.chromium.org/3201002

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