Chrome OS, Android, etc. do nothing to "widen the moat" (other than maybe some name recognition).
Android definitely widens the moat. It increases the value of the Google brand and prevents competitors like Bing from stealing market share in the mobile search and advertising.
Buffett talked about the value of Disney before the mid 90s when they cheapened the brand by releasing crap. When you bought a cartoon for you kids to watch, you know that the Disney movie will be quality even if you never heard of the movie before. No other cartoon studio could match Disney at the time.
Google has a similar moat as Disney used to have. People know that Google is a trustworthy company and has many quality products. Would you rather have a Gmail, Hotmail, Yahoo or AOL email address? How do you feel about using Yahoo or Bing as a search engine? How do you feel when you see a website that uses MapQuest to show its location compared to Google Maps? If you want to make money off online advertising, which company would you use?
When it comes to search and advertising, Google has a huge moat that cant be touched no matter how many billions of dollars Microsoft or Yahoo waste.