Move check for name-from:content out of getTextFromDescendantContent and...

... use getTextFromDescendantContent instead of getTextAlternatives in MainRoleOnInappropriateElement test

This was previously committed but got clobbered by a later commit.

Fixes bug reported here: https://groups.google.com/forum/#!category-topic/introduction-to-web-accessibility/ChGzBSe0_yU