wm: Run screenshot commands in background.
wm: Run screenshot commands in background.

It sounds like there was at least one case where the
screenshot command might've been hanging.  Not sure how
this could happen, but blocking the UI if it does is no
good.

BUG=chromium-os:4506
TEST=none
TBR=oshima

Review URL: http://codereview.chromium.org/2891002

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