Make GMail handle mailto: links in Google Chrome
Fortunately, Chrome does support Userscripts which makes hacking this in to your Chrome setup is no big deal. Over at the Chrome Plugins forums, one user has contributed a simple script [download] that automatically converts mailto: links to the GMail compose mail URL. Remember, Userscripts for Chrome go in your User Data\Default\User Scripts folder.
Further along in the forum, user PAEz contributes a Chrome-friendly .crx [download] extension for ultra-easy installation. Just download the file and click the install button when Chrome asks if you're sure you want to, and you're set. This version also defaults to opening your message in a new tab rather than a separate window.
Remember, depending on your version of Chrome you run and which iteration you decide to use you might need to add a switch to your Chrome shortcut. Right click and choose properties, and append --enable-extensions or --enable-user-scripts after chrome.exe on your shortcut's target.