Use document's contentSize when the HTML element is selected.

HTMLHtmlElement's size might not be the size of the document, such as
HTMLBodyElement's size, so when the selection root in the html element,
the best thing we can do is get the size of the content because <html>
and <body> might have a different size.

BUG=361927

Review URL: https://codereview.chromium.org/236083003

git-svn-id: svn://svn.chromium.org/blink/trunk@171441 bbb929c8-8fbe-4397-9dbb-9b2b20218538