Introduce LayoutObject::EnclosingBlockFlowFragment()

This patch introduces |LayoutObject::EnclosingBlockFlowFragment()| to share
logic for using inline node data among |LayoutNGBlockFlow::LocalVisualRectFor()|
and others for ease of adapting |LayoutInline| and |LayoutText|.

The patch[1] demonstrates usage pattern of this.

[1] http://crrev.com/c/795523: Adapt LayoutText::LinesBoundingBox() to LayoutNG

Bug: 591099
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I248cb213301cac6a92c713ca99b1450a9ff8a757
Reviewed-on: https://chromium-review.googlesource.com/799574
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520516}
3 files changed