[NGFragmentItem] Stop using rounded rect for link highlight when <a> has nested inline

This patch changes |LinkHighlightImpl::Paint()| to use squesr rect instead of
rounded rect when <a> has nested inline box, e.g. <a>ABC<b>DEF</b></a> as
same as |NGPaintFragment|.

This patch makes gesture-tapHighlight-simple-nested.html test to pass.

Bug: 982194
Change-Id: I824b6833223093892eb1aaa9936fa14c27c76028
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097777
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749101}