[LayoutNG] Convert physical fragments to being RefCounted.
A few changes in this patch:
NGPhysicalFragment becomes RefCounted.
NGFragment "views" becomes stack allocated.
(also remove an unused param).
NGFragmentBuilder moves off oil-pan to used being unique_ptr
(this one was done because the "build" step would have required
conversion from oilpan to non-oilpan code).
BUG=635619
Review-Url: https://codereview.chromium.org/2676533003
Cr-Commit-Position: refs/heads/master@{#448211}
32 files changed