Till now Chrome asks to enable or remove third-party extension installed by Windows programs silently, this will no longer be case starting from January 2014 onwards, Chrome on Windows on Stable an beta channels strictly won’t allow extensions install outside from Chrome Web Store. You just can’t drag and drop extension from other sites onto Chrome://extensions page for install, instead you may need to rename extension to zip file and need to upload from "Developer mode" to install it.

Chrome alerts for silently installed extension on Windows

Chrome on Windows strictly won’t allow extensions out side of web store

Why Google is making tough to install external extensions?

Google has the answer. "Many services bundle useful companion extensions, which causes Chrome to ask whether you want to install them (or not). However, bad actors have abused this mechanism, bypassing the prompt to silently install malicious extensions that override browser settings and alter the user experience in undesired ways, such as replacing the New Tab Page without approval. In fact, this is a leading cause of complaints from  our Windows users. "

If your extensions hosted elsewhere Chrome team suggesting you to migrate them to web store as soon as possible. Extension developers may also need to look at inline install feature.