Add WebDocument::scrollOffset, minimumScrollOffset, and maximumScrollOffset.

This CL is a preparation to support AXObject in PagePopup.

content/renderer/accessibility/blink_ax_tree_source.cc depends on
WebLocalFrame::scrollOffset, minimumScrollOffset, and
maximumScrollOffset now. However it's hard to provide WebLocalFrame
for PagePopup. We'd like to remove the WebLocalFrame dependency
with these new functions.


BUG=123896

Review URL: https://codereview.chromium.org/502643004

git-svn-id: svn://svn.chromium.org/blink/trunk@180910 bbb929c8-8fbe-4397-9dbb-9b2b20218538