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Using two fingers was not restricted which brought a bunch of other bugs.
Only allow one item to be dragged at a time. Still allows switching of
fingers midway, so we just check the item which requests drag events is
the same as the current drag item, instead of checking the gesture
finger id.

Test: ash_unittests WindowSelectorTest.DraggingWithTwoFingers
Bug: 827435
Change-Id: I2c897b1b3c483b5df7a98a658f575a6ebd706acd
Reviewed-on: https://chromium-review.googlesource.com/988294
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547932}