Upgrade html2canvas to remove XSS

* Upgrade html2canvas to v0.4.1
* This removes an XSS in the current version, along with various other
  upstream changes.
* I confirmed that the screenshot rendering still works.  It's better
  in some ways (e.g. gradients), though there's at least one new issue
  (search icon and 'Go' both render on top of each other).

Change-Id: I257c1e6bbba396fc341775b637128a770db52b71