[Unittest] Cover UIString.serializeUIString with test
[Unittest] Cover UIString.serializeUIString with test

Bug: 1009443
Change-Id: Ib347b63cc2a9d2197d725040903edc03e7fd6b88
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2046551
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend