At the moment, it isn't possible to set Chrome as the default external browser in Eclipse for Mac. This is because Eclipse uses the system's default browser, and on Mac, that is Safari. It is possible to change the default browser in Mac's System Preferences, but doing so will make Safari the default for all applications, not just Eclipse.
There are a few workaround methods that have been proposed:
One method is to create a custom browser in Eclipse and set the path to the Chrome executable. Instructions for doing this can be found here:
Another method is to create an Automator workflow that will launch Chrome when Eclipse is opened. Instructions for doing this can be found here:
Both of these methods require a bit of work to set up, but once they are in place, they should allow you to use Chrome as your default browser when launched from Eclipse on your Mac.