Updating plugin widget geometry every time we paint means we don't have to
explicitly request plugin geometry updates.
This patch stops us from flushing plugin geometry changes in
FlushPendingNotifications(Flush_Layout). There are too many Flush_Layouts and
flushing plugin geometry changes on them produces frequent
desynchronization of the plugin geometry with the rendered window contents.
There is some Web compatibility risk there --- it means we have to change
our tests, for one thing --- but hopefully it's OK.
--HG--
extra : rebase_source : 87adde45795ea2cab362ed9df54e62c5cc97e16c
d5a0e1a420