Using @custom-browser Cucumber tag instead of @user_agent

Implementing the convention:

https://www.mediawiki.org/wiki/Manual:Coding_conventions/Selenium#Option
al_tags

Bug: 61458
Change-Id: Iae1e40e04c0b993a45c1e46cd971c183b483bc70
Merge "Using @custom-browser Cucumber tag instead of @user_agent"