Use root window coords in EventGenerator constructor, screen coords elsewhere.

This feels inconsistent to me, but seems to be the best match for existing
practice and API documentation.  The other choice would be to use screen coords
everywhere, but this will require fixing up more tests.

This is a necessary precursor to using the configured screen position client by
default; right now, root window and screen coords are equivalent, so being
inconsistent about this doesn't matter.

One side effect here is that when constructed with a root window but no specific
starting point, the event generator's initial position will now be the origin of
the root window instead of the origin of the screen.

Bug: none
Change-Id: I9b9138a2fbd44ddf5c585f04db9927c4f052eb00
Reviewed-on: https://chromium-review.googlesource.com/c/1347587
Reviewed-by: Scott Violet <[email protected]>
Commit-Queue: Peter Kasting <[email protected]>
Cr-Commit-Position: refs/heads/master@{#611154}
40 files changed