Implement closed shadow adjustment for Element.offsetParent

If |offsetParent| is not an unclosed node of the context
object, search the layout tree up until an unclosed node
is found.

|offsetTop| etc. will be adjusted relative to this
|offsetParent| value.

See also
https://github.com/w3c/webcomponents/issues/497
https://github.com/w3c/csswg-drafts/issues/159
for the discussions about spec clarifications.

BUG=531990

Review-Url: https://codereview.chromium.org/2051703002
Cr-Commit-Position: refs/heads/master@{#402757}
22 files changed