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Previously, for double-click partial selections, the partial select
took place on mousedown, and the unelision took place on mouseup.

This CL makes them both take place on mousedown, to (hopefully) reduce
the visual noise.

Bug: 797354
Change-Id: I7a7d26740ffeef028a368a9d3b467c54674b8a83
Reviewed-on: https://chromium-review.googlesource.com/935828Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538979}