Bump the X priority to -20
Bump the X priority to -20

After some tracing, we noticed that the X event delivery is less janky when
X can run right away. This is especially critical for things like getbuffers
requests. To fix it, we bump X's priority.

BUG=chromium:120729
TEST=by hand, checked that the priority is -20

Change-Id: If712e48dea8d9593db8dbb766434e2491b8a8a80
Reviewed-on: https://gerrit.chromium.org/gerrit/19332
Tested-by: Stéphane Marchesin <e16f69845633a0cf74affe1e5a677caa6296c937@chromium.org>
Reviewed-by: Antoine Labour <9b5adc77717fb53f6e082b29cd49aa0d7f0ce7f0@chromium.org>
Reviewed-by: Nathaniel Duca <9466944aa6f67d8ab7d9861eb4c5c9c7494e3a7f@chromium.org>
Commit-Ready: Stéphane Marchesin <e16f69845633a0cf74affe1e5a677caa6296c937@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager