[android] Fire accessibility events when scrolling sublayers.

Cherry-pick http://crrev.com/48973004

> This enables the Android WebView to fire accessibility events for
> sub-layer scrolling. Currently this is only limited to touch-driven
> scrolling (JavaScript-initiated scrolling only works for the root
> layer at the moment).
> BUG=312318
> android-only change, trybots are happy
> NOTRY=true
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=232097

BUG: 10969501
Change-Id: I53fef2b655ff5e77e0924c4edc529e44f5b626ec
8 files changed