This CL addresses the following issues of layout shift tracking:
- Changed visual overflows triggered layout shift report
  (originally filed as crbug.com/1090763).
- Descendant layout overflows were not included in ancestor's layout
  shift report.
- Unnecessary descendant layout shift report.
- Slow visual rect calculation for LayoutInline and LayoutText.

It includes the following changes
- Track LayoutBoxes and LayoutTexts only.
- Track both old and new starting points and visual representations
  instead of using location of visual rects as starting points.
- For a LayoutBox, the local visual representation is the layout
  overflow rect if the LayoutBox has layout overflow and doesn't clip
  overflow. Otherwise use the border box rect.
- For a LayoutText, suppose its shift causes shift of all contents
  in containing block logically below the LayoutText, and use that
  part of visual representation of the containing block as the visual
  representation of the shift.
- Map both local starting points and local visual representations to
  the initial containing block and viewport.

This has the following benefits:
- More accurate starting points and visual representations, conforming
  to the specs;
- Better performance by
  * avoiding unnecessary layout shift reports of descendants
  * avoiding slow LayoutInline and LayoutText visual rect calculation.
- sizeof(LayoutObject) is reduced by 16 bytes because now we use the
  previous geometry data in LayoutBoxes for paint invalidation.

Bug: 1108622
Change-Id: I7ebb4e250e7ca2a03b4dadea50034e300e55ad84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319538
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793806}