use pageswap callback to sync WebTextView with drawing of page

bug:5008210

Redraw the WebTextView when the page swaps (and thus the background is updated)
That way it updates at (approximately) the same time as the page. In software
rendering, do as before.

Depends on following external/webkit change:
https://android-git.corp.google.com/g/#change,124606

Change-Id: I59d7266a49d0ba734efdb88fef5fafe68aecfbce