Followup fixes for bug 332066.

* <use> elements only dispatch load if there's an external resource,
  adjust tests accordingly.
* <use> incorrectly reported that it wasn't done if there was no
  xlink:href attribute.
* don't dispatch 'load' to svg documents that are not xml-wellformed.

BUG=333242
R=pdr@chromium.org, schenney@chromium.org

Review URL: https://codereview.chromium.org/134263003

git-svn-id: svn://svn.chromium.org/blink/trunk@165413 bbb929c8-8fbe-4397-9dbb-9b2b20218538