Revision 1005078

Backport r1002770 by tmcguire from trunk to the 4.3 branch:

Also support multipart/alternative with text/plain and multipart/related as children, instead
of the usual text/plain and text/html.

This fixes that the content of some Newsletters with images was rendered twice, bot the HTML and
the plain text part was shown.

CCBUG: 187989