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This fixes fragment data LogicalTopInFlowThread for overflowing contents
under clip under multicol. Previously the overflowing parts created
phantom fragments as if they were overflow:visible. Now the overflowing
parts are under the last fragment of the clipping element. They will be
painted correctly when they scrolls into the container clip rect.

Bug: 1063043, 1064015

Change-Id: Ie8d60250736b0a5074872e7ef8d021c5731f9b40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132934Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756142}