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This is a preparation to support AXObject in PagePopup.
PagePopup doesn't have WebLocalFrame, and it's difficult to provide it. We can
remove WebLocalFrame dependency of BlinkAXTreeSource by using
WebDocument::scrollOffset, minimumScrollOffset, and maximumScrollOffset.

This CL has no functionality changes.

BUG=123896

Review URL: https://codereview.chromium.org/510123003

Cr-Commit-Position: refs/heads/master@{#292839}