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Following CL:919443, this patch implements list-item further for
block-level content, and unifies the positioning logic with the
logic for inline content.

The anonymous blocks that wrap list markers were not easy that
this patch changes the tree building (LayoutBlockFlow::AddChild)
to handle LayoutNGListMarker as out-of-flow. However, setting
the out-of-flow properties (e.g., position: absolute) affects
many parts of the code base that it is limited only for the
tree building purpose.

Also, in order for the list item block to position its list
marker, NGInlineLayoutAlgorithm no longer positions it. Instead,
the |unpositioned_list_marker_| is moved to NGLayoutResult,
which propagates from line box to the nearest list item block.

Other logic are still in NGInlineLayoutAlgorithm because list
markers forces the line box not empty.

NGBoxFragmentPainter needs a minor change because by adding
list markers to list items, it may appear along with line boxes.

20 tests turned to pass, including some rebaselines for text
rendering differences. 4 tests regressed, that are
1. 'display: contents'; the tree building change needs to take
   this into account.
2. When inside/outside are changed from JS, we fail to compute
   the baseline.
I will investigate them, along with tests that still don't pass
in following patches.

Bug: 725277
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_layout_ng;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I09eb80b3a24a17b8841b4b200ff5d4a249c81853
Reviewed-on: https://chromium-review.googlesource.com/925381Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539144}