[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