[LayoutNG] Fix text fragment height not to include leadings

The size and offset of NGPhysicalTextFragment included the leadings
from the 'line-height' property. The size of text boxes are not
defined in the spec, but pre-NG InlineTextBox does not include it,
and using the same offset and size helps sharing the logic with
pre-NG.

Bug: 636993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I6fab82bf16e17da15306c14a04e690460a7f5712
Reviewed-on: https://chromium-review.googlesource.com/781482


Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518307}