Visualforce: Access to Page Metrics and Improved Security

Develop apps to customize your org using Visualforce. Changes in this release are focused on giving you access to Visualforce page metrics and improving the security of your pages. This change applies to Lightning Experience, Salesforce Classic, and all versions of the Salesforce1 mobile app.
Available in: Contact Manager, Group, Professional, Enterprise, Performance, Unlimited, and Developer Editions

For more information on features affected by these enhancements, refer to the Visualforce Developer Guide.