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_unittest.cc

@@ -8,10 +8,14 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include <vector>
-#include "webrtc/base/gunit.h"
 #include "webrtc/base/json.h"
 
+#include <vector>
+
+#include "webrtc/base/gunit.h"
+
+namespace rtc {
+
 static Json::Value in_s("foo");
 static Json::Value in_sn("99");
 static Json::Value in_si("-99");
@@ -275,3 +279,5 @@ TEST(JsonTest, DoubleVectorToFromArray) {
     EXPECT_EQ(in[i], outj[i]);
   }
 }
+
+}  // namespace rtc