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Based on the discussion in https://crrev.com/c/1743869, we decided to
add autotestPrivate.swapWindowsInSplitView() instead of using automation
API in Tast.

Tast side CL: https://crrev.com/c/1716679

TEST=arc.SplitView
BUG=135150701

Change-Id: I9e96b01daf0c8c84dc77363727dd4e38244562c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758037Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima (slow) <oshima@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689581}