[LayoutNG] Make LayoutNGMixin::PaintFragment const

PaintFragment should be mostly immutable.
Making LayoutNGMixin::PaintFragment() a const enforces this.

Bug: 635619
Change-Id: Idf2d56fb9bcb69e9e89510f6044cad6b280352cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620970
Commit-Queue: Aleks Totic <atotic@chromium.org>
Auto-Submit: Aleks Totic <atotic@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661519}
13 files changed