Ensure variables are decoded to unicode.
Ensure variables are decoded to unicode.

Add asserts to ensure variables are unicode. Otherwise it could get into a
situation where variables are left as str and this would cause an assertion.

R=csharp@chromium.org
BUG=

Review URL: https://codereview.appspot.com/39100045

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py