[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..cc86b857e937e93aaa60f1353703dd8b139f4b47 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 CanCaptureVideo(self):
+    return False
+
+  def StartVideoCapture(self, min_bitrate_mbps):
+    raise NotImplementedError()
+
+  def StopVideoCapture(self):
+    raise NotImplementedError()