Implemented UI for new first-run tutorial.
* Added new methods to ash::FirstRunHelper.
* Implemented tutorial flow in FirstRunController.
* Added base class Step and implemented for steps of tutorial.
* Added string resources and assets.
* Tutorial works correctly for RTL languages.
Knowing issues:
* Buttons "Keep exploring" and "X" don't work yet.
BUG=269286
Review URL: https://codereview.chromium.org/
46293002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233131 0039d316-1c4b-4281-b951-d872f2087c98