Add text extensions to the extension mime type map.

The Browser used to have a concern around mapping text extensions to various
text/* mime types. If a text extension mapped to an unknown text/* mime type,
the Browser would attempt to download a url instead of just displaying the
content.

There is another change in external/webkit that uses 2 static functions in
DOMImplementation that determine if a mime type is text or xml. The text
function in particular checks for a leading "text/" string in the mime type.
With that change, pruning text mime types is no longer needed.

Bug: 1988375