[Telemetry] Refactoring in preparation for video-based Speed Index support.

tools/telemetry/telemetry/core/platform/platform_backend.py

Index: tools/telemetry/telemetry/core/platform/platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/platform_backend.py b/tools/telemetry/telemetry/core/platform/platform_backend.py
index 7f74deabf1978ca302bea45219da4acb30157d51..29b067f08865c1d864dfe5461d2085aa853b7843 100644
--- a/tools/telemetry/telemetry/core/platform/platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/platform_backend.py
@@ -75,3 +75,12 @@ class PlatformBackend(object):
 
   def InstallApplication(self, application):
     raise NotImplementedError()
+
+  def CanCaptureBrowserVideo(self):
+    return False
+
+  def StartBrowserVideoCapture(self, tab, max_bitrate_mbps):
+    raise NotImplementedError()
+
+  def StopBrowserVideoCapture(self):
+    raise NotImplementedError()