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Currently, DragDescriptor is a simple class with a bunch of getters.
The C++ style guide suggests that DragDescriptor is better off as a
struct.

Change-Id: I0b982aea715627226bb2cd3ff924cde0d3c61f7c
Reviewed-on: https://chromium-review.googlesource.com/1056942Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558893}