Extracted the X11DragDropClient tests.

This CL is one more step towards unification of classes used on Linux
with X11.

The lower level of the drag and drop on X11 is handling the XDND
protocol, which is done by ui::XDragDropClient; the latter is currently
used for both non-Ozone and Ozone paths, but historically it had been
part of DesktopDragDropClientAuraX11, so there were no tests for it.

Here DesktopDragDropClientAuraX11 tests that in fact test the XDND level
are moved out and test ui::XDragDropClient directly.  The higher level
ones (like, for example, DesktopDragDropClientAuraX11Test.HighDPI200
that tests that the drag widget is moved properly on different scales)
are subject to move to DesktopDragDropClientOzone tests.

Note: a better place for these tests would be perhaps x11_unittests but
they cannot be moved there easily.  We keep that in mind and will try to
address that later.

Bug: 990756
Change-Id: If1905b2abace0db72a6aa8e4d0f0834bdafded81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250104
Commit-Queue: Alexander Dunaev <adunaev@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780400}

ui/views/BUILD.gn

@@ -1277,6 +1277,7 @@ test("views_unittests") {
         sources += [
           "widget/desktop_aura/desktop_drag_drop_client_aurax11_unittest.cc",
           "widget/desktop_aura/desktop_screen_x11_unittest.cc",
+          "widget/desktop_aura/x11_drag_drop_client_unittest.cc",
         ]
         deps += [ "//ui/base/x:test_support" ]
       }