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This will make notification latched at an in-between position by
partially sliding it left or right, when the notification has either or
both snooze/settings buttons. The position to latch the slider changes
based on the number of those buttons.
This change does not yet show buttons there.

The feature is behind a flag.
To test this, add --enable-features=NotificationSwipeControl flag.

Bug: 840497
Change-Id: I5fce9216ebae79112d66eb289f6e669347c4aba7
Reviewed-on: https://chromium-review.googlesource.com/1177464Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585090}