The chrome.tabCapture API is available for Chrome apps and extensions.
This makes it possible to capture the visible area of the tab as a stream which can be used locally or shared via RTCPeerConnection’s addStream().
For more information see the WebRTC Tab Content Capture proposal.
Screensharing was initially supported for ‘normal’ web pages using getUserMedia with the chromeMediaSource constraint – but this has been disallowed.
EDIT 1 April 2015: Edited now that screen sharing is only supported by Chrome in Chrome apps and extensions.