Implement display:flow-root

display:flow-root generates a block container box, and lays out its contents using
flow layout. It always establishes a new block formatting context for its contents.

Intent to implement issued at:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/k9WOAVA5Ewc

Test=display-flow-root-001.html taken from the Mozilla test-suite

https://www.chromestatus.com/features/5769454877147136

TBR=pfeldman@chromium.org
BUG=672508

Review-Url: https://codereview.chromium.org/2620623002
Cr-Commit-Position: refs/heads/master@{#445258}
12 files changed