Add support for Safari Technology Preview 34 and High Sierra Beta

[marca56]

Safari for Mac and IOS are currently missing VP8 support but most WebRTC tests pass with H.264 enabled:

• Enhance SDP in Olympus to use H.264 and fallback to VP8 and VP9
• Check whether Chrome H.264 is enabled; if not, show a warning notification "Please enable H.264 in your browser"

Here's more information about Chrome support for H.264: http://www.rtc.news/posts/pawtYYfpmsG6Zed3W/post.getPageUrl(true)