[Telemetry] Explicitly define Name() method for all perf benchmarks

tools/perf/benchmarks/webrtc.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from measurements import webrtc
 import page_sets
 from telemetry import benchmark
 
 
 class WebRTC(benchmark.Benchmark):
   """Obtains WebRTC metrics for a real-time video tests."""
   test = webrtc.WebRTC
   page_set = page_sets.WebrtcCasesPageSet
+  @classmethod
+  def Name(cls):
+    return 'webrtc.webrtc_cases'
+