handle scrolling by Y nodePosition

[michelson]

TODO:

• fix 1 failing spec

Summary

tackles issue #304

Test Plan

When you have a big paragraph and you press enter in the middle, instead of keeping the focus in the middle of the page, the editor scrolls all the way to the bottom.

Steps to reproduce bug in current draft :

• Go to https://facebook.github.io/draft-js/
• Copy paste in a large continuous paragraph without any newlines in between.
• Scroll to the middle of the big paragraph and press enter.
• The editor will have scrolled to the bottom
• This doesn't seem to happen consistently all the time, but I've seen it come up enough times on big paragraphs.

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[rezaaa]

What happened to this issue?

[dtmunir]

+1 on getting this reviewed and merged

[erisnuts]

+1

[niveditc]

Superseded by #1223 which seems to have a better fix as per the discussion on #304. Waiting for conflicts to be resolved to merge in #1223, and closing this PR.