[Harmony Cast Dialog] Store and restore scroll position before and after update

Instead of storing the scroll position only when the main button is clicked,
store the position right before each sinks update, so that the scroll position
is retained across updates.

Bug: 852851
Change-Id: I0052e55eec1de532987dbc211ac04496b67b847d
Reviewed-on: https://chromium-review.googlesource.com/1137345
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576926}
1 file changed