From ars technica:
Google has also added some new security functionality to Chrome. Every time that the user downloads a file, the browser will compare it against a whiltelist of known-good files and publishers. If the file isn’t in the whitelist, its URL will be transmitted to Google’s servers, which will perform an automatic analysis and attempt to guess if the file is malicious based on various factors like the trustworthiness of its source. If the file is deemed a potential risk, the user will receive a warning.
Google says that data collected by the browser for the malware detection feature is only used to flag malicious files and isn’t used for any other purpose. The company will retain the IP address of the user and other metadata for a period of two weeks, at which point all of the data except the URL of the file will be purged from Google’s databases.
Users who are concerned about the privacy implications of this functionality can prevent the browser from relaying this information to Google by disabling the phishing and malware protection features in the browser’s preferences. You can refer to the official Chromium blog for additional details about the malware detection feature.