[LayoutNG] Introduce NGPhysicalFragment and make NGFragment a 'view'

- Adds a bunch of conversion functions between physical and logical offsets and sizes.
- The NGAlgorithm now outputs a NGPhysicalFragment (NGFragment output doesn't make sense in this instance as would immediately get unwrapped and destroyed).

BUG=635619

Review-Url: https://codereview.chromium.org/2282213002
Cr-Commit-Position: refs/heads/master@{#415543}
27 files changed