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This patch changes |ShouldCreateBoxFragment()| to return |true| for producing
box fragments for plain SPAN when |LayoutNGPaintFragments| runtime feature
enabled, to locate |NGPhysicalFragment| associated to |LayoutInlinline| for
computing bounding box and other kinds rects.

Without this patch, we need to traverse fragment tree rooted by
|LayoutBlockFlow| containing |LayoutInline|.

In legacy layout tree, we don't generate |InlineBox| for |LayoutInline| for
performance optimization and called "Culled InlineBox". To compute rects in
legacy layout tree, we traverse layout tree and line boxes in |LayoutInline|.
This code makes |LayoutInline| implementation complicated. Note: There is a
flag disabling "Culled InlineBox" as |AlwaysCreateLineBoxesForLayoutInline|.

Once LayoutNG is stable enough to have profiling, we investigate whether
generating fragment for every SPAN causes performance regression or not.


[1] http://crrev.com/c/763188: ONE [LayoutNG] Utilize DCHECK(CanUseInlineBox())
in core/layout/

Bug: 789390
Change-Id: Ife87fc9ee487397891ae4515d1bcaea1afcf9c04
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Reviewed-on: https://chromium-review.googlesource.com/790177
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520865}