Two years back, Google has ported “OK Google” hotword Voice Search feature in Android version to desktop Chrome browser in the form of an extension called Google Voice Search Hotword beta, later the extension has been integrated into the browser settings.
You can enable ‘OK Google’ in Settings to start a voice search on Google.com or new tab. You need to allow Chrome to use your microphone, if you deny (block) you’ll get ‘voice search has been turned off’ error when you click on the mic again.
‘OK Google’ feature removed from Chrome 46
Recently due to privacy concerns raised by users over hotword behavior in Chromium 43, Google has removed the hotword component from Chromium 45, and now from Chrome 46 (Windows, Mac and Linux) as well, cited reason as low usage.
Voice Search is still available on google.com homepage and new tab when you click on mic icon. This feature is still available on Chromebooks.
This is what Google said on this change
“In Chrome 46, we’ll be removing the ability to use the “OK Google” voice action on Chrome for desktop as we haven’t seen many users utilize this feature on these platforms. Voice search will still be available on the New Tab Page and for any google.com pages when you click on the mic icon. Note that this feature is still available on Chromebooks”