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This CL is a part of changes for 'Drag and Drop' on wayland and
extends WaylandDataDevice, WaylandDataSource and WaylandDataOffer.

WaylandDataDevice registers event listeners for 'drag and drop',
has 'StartDrag' API connected with 'wl_data_device_start_drag' and
includes reading the data from Wayland.

WaylandDataSource registers event listeners for 'drag and drop' to
handle at a source side and writes the data to the fd delivered
from Wayland when it gets 'OnSend' event.

WaylandDataOffer gets the operation decided by Wayland and requests
the offered data by Wayland.

FakeServer is also extended to support the various compositor side
functionalities needed so that tests 'drag-and-drop' in
wayland_data_device_unittest.cc run. MockDataDevice, MockDataOffer
and MockDataSource classes are extended, implementing the respective
Wayland compositor hooks to communicate with Wayland clients
(eg Ozone/Wayland).

Flow:
1) Sending dragging
'WaylandDataDevice::StartDrag' triggers the drag action to Wayland,
Wayland requests the data through 'WaylandDataSource::OnSend' and
'WaylandDataSource' listens the status through 'OnCancel' or
'OnDnDDropPerformed'.

2) Getting dragging
'WaylandDataDevice' listens drag-and-drop status such as entered/
moved/dropped/left. When the drop action is performed,
'WaylandDataDevice' reads the data using 'WaylandDataOffer'.

BUG=875164
TEST=ozone_unittests

Change-Id: Ic1ad80fce4ac396385f3ba6780839ef31d717683
Reviewed-on: https://chromium-review.googlesource.com/1176891Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#589375}