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We need to reset the scrolling status (especially LayerTreeHostImpl::
did_lock_scrolling_layer_) when activating the pending tree in which
the currently scrolling node no longer exists.

Previously the situation caused DCHECK failure.

Change-Id: I23b9105ded1767ee27fa37fb75f67c13e81d5699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1882301Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710530}