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Also changes what happens when the drag ends during a nested move loop.  It
now exits the move loop (which triggers a drag end) instead of immediately
ending the drag.

This is the first step in a larger refactoring for TabDragController:
go/tabdragcontroller-untangling

Change-Id: I6cabc5237a604320c934bb8b5d4bbc9ff6d231f0
Reviewed-on: https://chromium-review.googlesource.com/c/1285085
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611352}