Fix behavior of Document.body getter

According to [1], the body element is the first child of the html
element that is either a body element or a frameset element. The
current version does not check that the parent node is the html
element, so add that check.

Behavior matches Firefox.

[1] https://html.spec.whatwg.org/multipage/dom.html#the-body-element-2

BUG=490511

Review URL: https://codereview.chromium.org/1578193006

Cr-Commit-Position: refs/heads/master@{#369654}
2 files changed