Increase coverage of telemetry's user_agent_unittest

Added a mobile and desktop testing in case the browser being tested has a
user agent that defaults to the tablet user agent.

BUG=421229

Review URL: https://codereview.chromium.org/640453002

Cr-Commit-Position: refs/heads/master@{#300349}
diff --git a/AUTHORS b/AUTHORS
index a6a5491a..a61c7a6 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -96,6 +96,7 @@
 Daegyu Lee <[email protected]>
 Dai Chunyang <[email protected]>
 Daniel Bomar <[email protected]>
+Daniel Imms <[email protected]>
 Daniel Johnson <[email protected]>
 Daniel Nishi <[email protected]>
 Daniel Shaulov <[email protected]>