[LayoutNG] Remove NGPaintFragment selection invalidation logic

Now we are (maybe temporarily) using the containing LayoutObject's
visual rect for NGPaintFragments, and we have included selection
into LayoutText's visual rect (crrev.com/c/1457812), so we can
remove the NGPaintFragment selection invalidation logic to avoid
the extra cost and complexity.

Bug: 636993
Change-Id: Ia87ee6a25b3ad78ddb4e365daaddeddf43bfbb70
Reviewed-on: https://chromium-review.googlesource.com/c/1461184
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631074}
18 files changed