Paraphrasing spec from https://docs.google.com/document/d/1D27SB2VV8Nv4pTAZmtiR-Yt_nh9LKMdthwyYaKpVkCE

1. Top-Chrome automatically hides when the user touch-scrolls the page.
2. We only track the root frame of the web page. Scrolling embedded frames or other content does nothing.
3. As the page contents move up (because the user is moving their finger up) the top-chrome moves off-screen at a 1:1 ratio. 
4. If the user stops scrolling while top-chrome is partially visible, then top-chrome will animate-scroll either off-screen (iff we've passed the threshold, currently defined as 75% hidden) or back on-screen (if we haven't passed the threshold)
5. The opposite happens if the user scrolls down when top-chrome is hidden (including the transition thresholds for partial scrolling)
6. For the initial release, we only hide chrome on maximized browsers.

Please implement behind a flag, since this hasn't hit UI review yet.

Marking as Available if anyone is interested in poaching.

UX has recommended we block this work until they can staff it. Discussion here: https://docs.google.com/a/google.com/document/d/1D27SB2VV8Nv4pTAZmtiR-Yt_nh9LKMdthwyYaKpVkCE/edit?disco=AAAABNZs0j8

duplicate? https://bugs.chromium.org/p/chromium/issues/detail?id=867063 (I don't have permission to see it, I just found it from this https://9to5google.com/2018/10/03/chrome-os-scrolling-browser-ui/ )