XMLSerializer.serializeToString() should throw exception for invalid node value.

FF and Opera 12 (pre blink/webkit) throw exception in case the node value is invalid.
Blink sends an empty string as the result of serializeToString operation with invalid node.
We should match with other browsers for throwing it as an exception.

R=eseidel, abarth

Review URL: https://chromiumcodereview.appspot.com/23532055

git-svn-id: svn://svn.chromium.org/blink/trunk@157617 bbb929c8-8fbe-4397-9dbb-9b2b20218538