TraceProfiler categories_with_flow +=

Review URL: https://codereview.chromium.org/1147893002

Cr-Commit-Position: refs/heads/master@{#330769}

tools/telemetry/telemetry/core/platform/profiler/trace_profiler.py

@@ -21,7 +21,7 @@ class TraceProfiler(profiler.Profiler):
     # We always want flow events when tracing via telemetry.
     categories_with_flow = 'disabled-by-default-toplevel.flow'
     if categories:
-      categories_with_flow = ',%s' % categories
+      categories_with_flow += ',%s' % categories
     options = tracing_options.TracingOptions()
     options.enable_chrome_trace = True
     self._browser_backend.StartTracing(