[LayoutNG] Add LayoutObject pointer to each NGPhysicalFragment.

This pointer is for:
 1) Copying back data to the legacy layout tree inside NGBlockNode.
 2) When the legacy layout tree is using fragments for geometry
    information this will be used while walking the fragment tree to
    access the layout object which generated the fragment.

BUG=635619

Review-Url: https://codereview.chromium.org/2653773004
Cr-Commit-Position: refs/heads/master@{#445932}
21 files changed