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If a tab dragging is ended by a second finger tap down, PerformDeferredAttach()
might be called to merge the dragged tab into an overview window under the
current event position. The event position is calculated based on the current
attached tabstrip if it's a touch down event, thus we need to make sure the
current attached tabstrip is not nullptr when GetCursorScreenPoint() is called.

Bug: 877698
Change-Id: Ic2dcf9f2d4e79c00f3fd7ff0b724a8a45ba64781
Reviewed-on: https://chromium-review.googlesource.com/1200526Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588868}