7b3eca9940
Similar to the existing constants for the media constraint-keys intended for media sources, add the constants for the standard keys to generate offers and answers. This patch also adds a few comments to RTCMediaConstraints.h, to give a better clue on the intended usage scope of declared media-constraint keys and values. BUG=webrtc:7722 Review-Url: https://codereview.webrtc.org/2908013002 Cr-Commit-Position: refs/heads/master@{#18362}