Fix DCHECK failures in DesktopScreenWayland with single process mode.

When browser is launched in single process mode, during startupwayland
events are dispatched quite late. Thus Screen Geometry was set quite
late. This patch adds support for forcing flush in
DelayedChannelHostInitialization, this would ensure OutputHandleMode
is dispatched immediately.
Force display flush with multi process side.

In multi process case, we poll on a IO thread. The events are not
dispatched properly in certain cases like when weston is locked
during browser launch. This patch forces a flush in case the
OutputMode event wasn't dispatched during connection phase.