GitLab

In this CL:
- Always supply a user agent to WKWebViews in web//'s creation methods.
  This allows the IS_IOS variable in offline.js to be correctly set to
  true.
- Delay initializing the game runner until after images have been
  loaded.
- Check for existence of errorPageController when activating the game.

BUG=none

Review-Url: https://codereview.chromium.org/2144933002
Cr-Commit-Position: refs/heads/master@{#406949}