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This patch concerns so called external window mode,
which is being developed downstream now [1].

The problem this CL fixes is the following:
Even though events are passed to the right X11WindowOzone
based on an XID target now, there can be cases when the
events must be rerouted to another X11WindowOzone, for
example, a context menu or a nested 3-dot menu. In case of a
nested 3-dot menu, events must always be passed to the parent
menu, so that the menu controller would handle events properly.
Another case is handling events when a user clicks outside the
menu's area: even though the events can be targeted for another
window, there can be an explicit grab that must reroute them
to the menu window, which will be closed as soon as a user
clicks outside the menu's area.

The patch fixes the above mentioned problem the following way:

1) Once an explicit grab is set, the events are passed to an
X11WindowOzone have their locations converted and are rerouted
to a grabbed window.

2) Whenever a grabbed window looses an explicit capture,
X11WindowManager tells X11WindowOzone the capture is lost
and the |delegate_| is notified about that.

[1] https://github.com/Igalia/chromium

Bug: 707406
Change-Id: I1b00e79c8ddc17b000c2f55b7f39c9ea16c0c71a
Reviewed-on: https://chromium-review.googlesource.com/790773
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542927}