Ignore layout shifts for adding/removing position:fixed

- Treat position:fixed in scrolling LayoutView as layout shift root
  (by checking CompositingReason::kScrollDependentPosition, which
  also covers the previous sticky position condition).

- Check whether an object was a layout shift root at the
  beginning of PaintPropertyTreeBuilder::UpdateForSelf(). Previously
  was_layout_shift_root was incorrect if UpdateFragments() removed
  PaintProperties() of the FragmentData.

- Update layout_shift_root_changed in both UpdateForSelf() and
  UpdateForChildren(), so that the flag also applies to the object
  itself if any paint properties in UpdateForSelf() affected
  layout shift root status.

Bug: 1150657
Change-Id: I74a5e3bab93e66483b47eb27a04e7bf2277ee1c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2665761
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849650}