save/restore conversation view scroll position

Save scroll position as a fraction of total height to handle
scaled and width-change cases. Restore scroll position fraction
in JavaScript as soon as the document geometry is known.
(This was how it was done in Gmail1, and I can't find a better
non-JS way to detect this signal.)

Does not restore scale upon rotate/re-render.

Bug: 7246054
Bug: 7022509
Change-Id: Ibf811b31ba6d7d79d34c236f6ffe2229d626528f
2 files changed