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Wayland Ozone backend uses a platform drag-and-drop session to be able
to support the capabilities required by Chrome's tab dragging.
Particularly, keeping track of the focused window and the current mouse
cursor position while the tab/window is dragged around. Further context
can be found at the design doc [1].

So far, it has been relying only on {Run,End}MoveLoop API to trigger
such DND session. It works, but, as it is called after the (dragged)
windows creation and mapping, it is not possible to take the necessary
actions upon window creation or mapping (e.g: issue some dnd/extension
protocol request to the Wayland compositor), which makes it insufficient
to achieve glitches-free tab/window detaching implementation. Which has
been verified while addressing [2][3], and can be seen at video [4].

To fix it, this patch does:

- Add WaylandExtension platform window extension, with a single method
for now, that makes it possible for the upper layer components to start
the window dragging session explicitly;
- Plumbs up Chrome's Browser{Frame,DWTHLinux} such that it starts a
Wayland drag-and-drop session when the tab drag starts.

[1] https://docs.google.com/document/d/1s6OwTi_WC-pS21WLGQYI39yw2m42ZlVolUXBclljXB4/edit?usp=sharing
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=1099418
[3] https://chromium-review.googlesource.com/c/chromium/src/+/2307653
[4] https://www.youtube.com/watch?v=3sWVyA1OagA

R=sky@chromium.org

Bug: 896640
Test: Covered by ozone_unittests
Change-Id: I5e902caa7b495ae585330bc3e3b431bacb139d1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346545
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#797778}