Improve the way that sections are loaded into the WebView.

* Do it using chained Runnables on the Java side.
* No longer use cascading messages between Java and JS bridge, since that
was prone to occasional stack overflow errors in JS, which would cause
weird/undefined behavior.
* Simplify scrolling to a specific section upon loading the page.
* Improve scrolling to specific y-offset after restoring page from state.

(To be continued with a patch that fixes scrolling to a specific section
from links clicked outside the app)

Bug: 69700
Change-Id: I10a5f1bd025ed77615847f3865ecf72a4e836890