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This patch changes |AbstractLineBox| to utilize |NGInlineCursor| to reduce usage
of |NGPhysicalLineBoxFragment| for prepration of migrating |NGFragmentItem|.

Note: To reduce patch size, following patch will introduce |NGInlineCursor|
version of |ClosestLeafChildForPoint()|.

Bug: 982194
Change-Id: I539b940509ae608f0068a013670889bfc16dd4ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877499
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709438}