Notebook isolated content: fix firefox extra-space issue

In Firefox, if the iframe initial height is set to 0, the reported
scrollHeight is too large. Workaround: set the initial height to 1.

IPython/html/static/notebook/js/outputarea.js

@@ -335,6 +335,7 b' var IPython = (function (IPython) {'
             // Create an iframe to isolate the subarea from the rest of the
             // document
             var iframe = $('<iframe/>');
+            iframe.attr('height', 1);
             iframe.attr('frameborder', 0);
             iframe.attr('scrolling', 'auto');