Update WebViewImpl::contentsPreferredMinimumSize to be agnostic to the scrolling element
Update WebViewImpl::contentsPreferredMinimumSize to be agnostic to the scrolling element

When the scrollingElement is the documentElement, documentElement.scrollHeight behaves
differently (and in unit tests may return 0).  So make sure that
contentsPreferredMinimumSize always uses the document element's layout box directly.

This is a no-op today, but will ensure the behavior of this API doesn't change
when we enable the scrollTopLeftInterop feature.

BUG=157855

Review URL: https://codereview.chromium.org/1291723005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200540 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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