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-Commit-Position: refs/heads/master@{#8921}

webrtc/base/json.h

@@ -20,7 +20,7 @@
 #include "third_party/jsoncpp/json.h"
 #endif
 
-// TODO: Move to rtc namespace
+namespace rtc {
 
 ///////////////////////////////////////////////////////////////////////////////
 // JSON Helpers
@@ -86,4 +86,6 @@ bool GetDoubleFromJsonObject(const Json::Value& in, const std::string& k,
 // Writes out a Json value as a string.
 std::string JsonValueToString(const Json::Value& json);
 
+}  // namespace rtc
+
 #endif  // WEBRTC_BASE_JSON_H_