Cleanup: Move json.h into rtc namespace.
Cleanup: Move json.h into rtc namespace.

This should fix the TODO in that header.

BUG=None
TEST=ninja -C out/Debug still compiles everything.
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47919004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8921}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cb76b895728c888c60cf474070fff6aee4f8731c

sippet/talk,sippet/talk,sippet/talk,sippet/talk,sippet/talk