Add chromium support

[srinivas1729]

I was using Dart SDK inside the Chrome OS Linux container. Instead of figuring out how to interact with chrome running outside the container, i just installed chromium in the container. Chromium appears to be able to handle Dart SDK's and webdev's needs. But this package only tries "google-chrome" in Linux. I added a local symlink /usr/local/bin/google-chrome -> /usr/bin/chromium and weddev is able to use chromium. But it would be nice if this package could directly use chromium on all platforms if it's available.

Note: I launch my web app inside Intellij with Dart plugin. So I didn't try setting CHROME_EXECUTABLE environment variable.