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For historical reasons, we modified the ARC framework to let each app
to report the cursor position relative to its window.

Now, the modification can be removed because we phased out Android-M based ARC.
More importantly, the modification *should* be removed, because it is not
compatible with third-party Android IMEs, and it caused a minor misplacement
in corner cases.

Note that we still need to keep the old window-relative mode, since ARC
notifications are still thinking themselves to be placed at (0,0), and thus
relying on this mode.

BUG=b/73097347
TEST=components_unittests ArcImeServiceTest
TEST=Manually on PlayStore, Settings, Hangouts (notifications),
  Dialog in Gallery KK" app, with and without multi-displays.

Change-Id: I8cd4581f4ccb5fb331c5686510b225527fea69f5
Reviewed-on: https://chromium-review.googlesource.com/902987Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Kazuhiro Inaba <kinaba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537882}