Google Chrome for iOS and Android have been using Data Compression Proxy for a while. This feature has been brought to desktop thanks to the open source Data Compression Proxy extension by niu tech. Now Google is developing its own extension entitled Data Saver (codenamed Flywheel Desktop), screenshot of which has recently leaked:
UPDATE: The official extension is now available in Chrome Web Store.
You can also compare Chrome Compression Proxy with Opera Turbo and Mozilla Janus.