Less duplicated code between nodeAtPoint() in LayoutBox and LayoutBlock.

This is a preparatory patch for moving line/float-specific parts of
LayoutBlock::hitTestChildren() into LayoutBlockFlow.

BUG=302024

Review-Url: https://codereview.chromium.org/1957673002
Cr-Commit-Position: refs/heads/master@{#392116}
4 files changed