Refactor LayoutTreeBuilderTraversal to expose a cleaner interface to layout sibling nodes.

This is the first step towards fixing the whitespace attachment bugs we have. My
plan is to use this from recalcDescendantStyles, and reattachWhitespaceSiblings
instead of the light tree.

BUG=657748

Review-Url: https://codereview.chromium.org/2725953002
Cr-Commit-Position: refs/heads/master@{#456105}
2 files changed