We are trying to build a webapp for videochat that will run in a browser across platforms (desktop, iOS, Android).
At this time we are not aware of how to run webRTC
There are frameworks like easyrtc theoretically allowing developers to write webRTC apps without needing to worry about browser-specific issues. However, webRTC is quite new and not completely stable, yet. As far as I know, webRTC does not work reliably for Chrome for iOS and not at all for Safari.
Update as of June 2017: WebRTC and getUserMedia are available in WebKit since iOS 11.
See also: Release notes from Safari 11.0
New in Safari 11.0 – Support for real-time communication using WebRTC.
New in Safari 11.0 – Camera and microphone access. Added support for the Media Capture API. Added ability for websites to access camera and microphone streams from a user's device (user permission is required.)