Delimit text nodes in debug observations

A DOM change can consist solely of splitting or joining text nodes. This can
have important effects in the browser (e.g. breaking Arabic ligatures) but is
completely invisible in HTML-serialized output.

This change uses modified HTML serialization, with each text node delimited
by <#text>...</#text> tags, to show the difference in debug observations.

Change-Id: If7e0de4845f73d75ce18244b7a205a1c6f7cec06
Merge "Delimit text nodes in debug observations"