We're not 100% sure how this feature is going to work, but the description makes it sound like Android L will be able to route WebView traffic through the same Google servers used to compress Chrome traffic in order to save data. That service streamlines code and swaps big images out for smaller webp files. That could be very useful if you run apps like Link Bubble to poke around on the web. It's a bit like Opera Max, which uses proxy compression on all unencrypted data system-wide (but this is WebView only).
Even assuming this feature is what it appears to be, there's no guarantee it works just yet. After all, it's an "experimental" feature in a developer preview build of Android. It might be incomplete or just plain broken.