[Telemetry][Tracing] Split StopTracing and CollectTraceData for Chrome Tracing Agent.

telemetry/telemetry/internal/backends/browser_backend.py

Index: telemetry/telemetry/internal/backends/browser_backend.py
diff --git a/telemetry/telemetry/internal/backends/browser_backend.py b/telemetry/telemetry/internal/backends/browser_backend.py
index aca5857e7364fa2510163c4474ca8492aea62a62..9b3aff9da0051e7bebeaabe159ede233b85799e4 100644
--- a/telemetry/telemetry/internal/backends/browser_backend.py
+++ b/telemetry/telemetry/internal/backends/browser_backend.py
@@ -102,7 +102,10 @@ class BrowserBackend(app_backend.AppBackend):
                    timeout=web_contents.DEFAULT_WEB_CONTENTS_TIMEOUT):
     raise NotImplementedError()
 
-  def StopTracing(self, trace_data_builder):
+  def StopTracing(self):
+    raise NotImplementedError()
+
+  def CollectTracingData(self, trace_data_builder):
     raise NotImplementedError()
 
   def Start(self):