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Previously it was in HomeLauncherGestureHandler. That means all eligible
windows were hidden before the drag actually started. Move it to its own
class DragWindowFromShelfController so that we only hide window when
drag actually starts. It will also be benificial for the class
DragWindowFromShelfController to be called from other caller.

Bug: 997885
Change-Id: I0cb8285c9505b48db76d4f5e5103ca24ebe1d90a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872804Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708008}