Bug 1089266 - Don't rely on lightweight themes in webapps. r=mfinkle

This papers over the issue by allowing the context menu to open.

However, the root cause is that context menus in webapps don't make a whole lot of sense -- for example, in my test I see:

      http://the/app/url...
      Share Link
      Open Link in New Tab
      Open Link in Private Tab
      Copy Link
      Bookmark Link
      Open With Music App

If you perform one of the tab actions, you'll see a toast offering to switch tab, and an intent handler overlay will fire!

I suggest filing a follow-up bug for someone with more familiarity with WebappImpl to address this.