I'm afraid I disagree with mouser on this one. I think Google set its goal higher than simply keeping its throne in the search (and search ads) field. Google wants to take the lead in the cloud computing arena, IMHO, and it wants its vast array of current and future services to succeed.
Let's face it, modern browsers (FF, Opera and IE) have pretty good performance already in terms of rendering regular web pages. And yet for web2.0 and cloud computing services to effectively compete with their desktop brethren, performance remains an issue.
By making Google Chrome open source, it welcome others to borrow its code. If that means its key technologies becoming de facto industry standard, and better performance -- probably followed by greater market share -- for its various cloud computing services, it's all the merrier for Google.
Just my 2 cents, of course. We never know until it's really out there.