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Tab reparenting task is used for custom tab or in multi window mode
to move a Tab object from an activity to another. This CL refactors
it into its own per-tab class to drive the task.

Bug: 995903

Change-Id: I5d9565765902a5669fdd9cb2c3b7439228a53598
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829618
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707160}