PSA: scoped_refptr is moving from rtc:: to webrtc::

[Harald Alvestrand]

As part of the long term goal of merging all of visible WebRTC code into the single namespace "webrtc::" (tracking bug: https://crbug.com/webrtc/7484), we're moving the "scoped_refptr" class from "rtc::" to "webrtc::".

This widely used smart pointer type (which is similar, but not equivalent, to std::shared_ptr) allows memory management by reference counting, and has wide usage in WebRTC's API.

The process of switching will take a while, and backwards compatible aliases will be left in place for a good while, but new code should use webrtc::shared_refptr, not rtc::shared_refptr.

Tracking bug: https://crbug.com/webrtc/15622

Harald