[telemetry] Enable flow events by default in telemetry when tracing.

BUG=

Review URL: https://codereview.chromium.org/168823005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251971 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -14,7 +14,11 @@ class TraceProfiler(profiler.Profiler):
     super(TraceProfiler, self).__init__(
         browser_backend, platform_backend, output_path, state)
     assert self._browser_backend.supports_tracing
-    self._browser_backend.StartTracing(categories, timeout=10)
+    # We always want flow events when tracing via telemetry.
+    categories_with_flow = 'disabled-by-default-toplevel.flow'
+    if categories:
+      categories_with_flow = ',%s' % categories
+    self._browser_backend.StartTracing(categories_with_flow, timeout=10)
 
   @classmethod
   def name(cls):