Editing wants to have node which layout object is AtomicInlineLevel(), not sure
about list marker and float.

In NGInlineNode::CollectInlines(), kObjectReplacementCharacter is added into
text content for
 - IsFloating()
 - IsAtomicInlineLevel() except for list marker


# IsAtomicInlineLevel: based on call sites of SetIsAtomicInlineLevel()
- LayoutBlock if display:inline
- LayoutRubyRun inherited from LayoutBlockFlow
- LayoutReplaced: subclasses are:
 * LayoutEmbeddedContent
 * LayoutHTMLCanvas
 * LayoutImage
 * LayoutSVGRoot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8dbf1e70fcef99f2d8df1bbf497b58b255791b7e

commit 8dbf1e70fcef99f2d8df1bbf497b58b255791b7e
Author: Xiaocheng Hu <xiaochengh@chromium.org>
Date: Fri Oct 20 00:43:35 2017

[LayoutNG] Change offset mapping annotation type to general LayoutObject

This patch is a preparation for supporting IMG and other atomic inlines
in offset mapping, for which we need to use general annotation type in
offset mapping builder instead of LayoutText.

Bug:  776272 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I0c9a24dc4ff97f1c839cb36f25dbef38e30cd4a8
Reviewed-on: https://chromium-review.googlesource.com/728925
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510272}
[modify] https://crrev.com/8dbf1e70fcef99f2d8df1bbf497b58b255791b7e/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.cc
[modify] https://crrev.com/8dbf1e70fcef99f2d8df1bbf497b58b255791b7e/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_builder.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6b2e4dd03964d5389c80aae678502bc7c3176e68

commit 6b2e4dd03964d5389c80aae678502bc7c3176e68
Author: Xiaocheng Hu <xiaochengh@chromium.org>
Date: Sat Oct 21 01:14:39 2017

[LayoutNG] Support atomic inlines in offset mapping

This patch adds support of atomic inlines to offset mapping, so that
DOM positions anchored at an atomic inline (e.g, IMG@BeforeAnchor) can
be mapped to/from an offset in the NGInlineNode canonical text.

See unit test NGInlineNodeOffsetMappingTest.ReplacedElement for sample usage.

Bug:  776272 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ifc929780085323de1bb413eb584438cd3f001806
Reviewed-on: https://chromium-review.googlesource.com/731184
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510631}
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_inline_items_builder.cc
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_inline_items_builder.h
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_inline_items_builder_test.cc
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_inline_node.cc
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_inline_node_offset_mapping_test.cc
[modify] https://crrev.com/6b2e4dd03964d5389c80aae678502bc7c3176e68/third_party/WebKit/Source/core/layout/ng/inline/ng_offset_mapping_result.h