Microsoft will end the support for Windows XP on April 8, 2014. Windows XP is already 12 years old, but it's still being used by almost a third of PC users, in part because of enterprise customers, and to Chinese users. But even though so many users are still using it, Microsoft doesn't seem interested in supporting it any longer, and in a few months will end all support for it.
What that means is that no security patches will be given for vulnerabilities anymore. If hackers discover new vulnerabilities after that date, they will never be fixed anymore, and anyone who uses Windows XP will be vulnerable to such an attack. While I'm sure a lot of these companies and users are planning to move away from Windows XP, possibly to Windows 7 or even Ubuntu (especially in China), most of them will probably wait until the very last moment, until it's not supported by Microsoft anymore, and only then start the transition, which could take another year or longer.
Google has a partial solution to that problem, by at least offering these users a secure browser until April 2015, which is a year longer than Microsoft is willing to support Windows XP and IE8 (the last version of IE on Windows XP).
But why would Google do this? Google potentially has a lot to gain here, because a lot of companies are stuck with IE, whether it's IE6 or IE7 or IE8, or even newer versions, because they've built their internal web apps around that browser and its dependencies, and it's very hard to convince them to move to something else. But if they want to stay on Windows XP a while longer, then they have no other choice than to go with Chrome, until they decide to move off Windows XP.
That's great news for Google because those same customers might just stick with Chrome afterwards, and maybe become interested in ChromeOS, Google Apps, and other Google services for enterprise. So just by supporting Windows XP a while longer, Google can gain a lot of important enterprise customers, that will probably stick to Chrome for another decade, just like they stuck with IE and Windows XP for so long. Overall it's a smart move by Google, with very little cost for them.