renamed kName to kUri and make it more const.
remove IsSupportedBy to reduce header dependency.
BUG=webrtc:1994
Review-Url: https://codereview.webrtc.org/2457783005
Cr-Commit-Position: refs/heads/master@{#14825}
Since this conversion is used in multiple place and extension seems
right place to keep it in.
BUG=webrtc:1994
NOTRY=true
Review-Url: https://codereview.webrtc.org/2272563010
Cr-Commit-Position: refs/heads/master@{#13934}