Use local pages for webrtc telemetry tests.

tools/perf/page_sets/webrtc_cases/datachannel_files/common.js

Index: tools/perf/page_sets/webrtc_cases/datachannel_files/common.js
diff --git a/tools/perf/page_sets/webrtc_cases/datachannel_files/common.js b/tools/perf/page_sets/webrtc_cases/datachannel_files/common.js
new file mode 100644
index 0000000000000000000000000000000000000000..6d033bf3ee009c7a25d548fd9718287be2e6e769
--- /dev/null
+++ b/tools/perf/page_sets/webrtc_cases/datachannel_files/common.js
@@ -0,0 +1,7 @@
+ /* exported trace */
+
+// Logging utility function.
+function trace(arg) {
+  var now = (window.performance.now() / 1000).toFixed(3);
+  console.log(now + ': ', arg);
+}