[layoutng] Add an NGFragmentBuilder

This patch does a few things:
- Adds and uses an NGFragmentBuilder to create fragments
- Code style/variable and function naming cleanup in NGFragment
- Use the structs from ng_units in NGFragment to store sizes

R=eae@chromium.org,ikilpatrick@chromium.org
BUG=635619
NOTRY=true

Review-Url: https://codereview.chromium.org/2270983002
Cr-Commit-Position: refs/heads/master@{#414139}
10 files changed