Let Node.textContent never throw

http://dom.spec.whatwg.org/#dom-node-textcontent

This is per the spec. The only case where it could previously have
thrown is if ContainerNode::checkAcceptChild returned false. I could
not find a situation where that would have happened, so just assert
that the internal appendChild call does not throw an exception.

BUG=326133
TEST=none

Review URL: https://codereview.chromium.org/106503002

git-svn-id: svn://svn.chromium.org/blink/trunk@163275 bbb929c8-8fbe-4397-9dbb-9b2b20218538