The current logic assumes that all spans in the text layer contain
only one text node, and thus that the position information
returned by `highlighter._convertMatches` can be directly used
on the element's only child.
This is not true in case of highlighted search results: they will be
injected in the DOM as `<span>` elements, causing the `<span>`s
in the text layer to have more than one child.
This patch fixes the problem by properly converting the (span, offset)
pair in a (textNode, offset) pair that points to the right text node.
b47d81536c