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Was trying to fix something in this code for an unrelated bug and came
across a few things that could be improved:
* There were too many stateful booleans making the code unnecessarily
complex.
* The type of event that initialized the drag should be in the
DragDescriptor, because it describes the drag and also has the same
life cycle.
* Flatten complicated if statements into chains of if/else's that only
use simple conditions.
* Use a switch statement for the event type.

Change-Id: I74712df2502960c6a98be0a705a5ced94a61ec75
Reviewed-on: https://chromium-review.googlesource.com/979378Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Blake O'Hare <blakeo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545737}