Add is_tracing to browser.

tools/telemetry/telemetry/core/web_contents.py

Index: tools/telemetry/telemetry/core/web_contents.py
diff --git a/tools/telemetry/telemetry/core/web_contents.py b/tools/telemetry/telemetry/core/web_contents.py
index 550103444d234706540362a30102980eef562106..4e29c7d5a7f7fc1164c00705555279b75f65eac4 100644
--- a/tools/telemetry/telemetry/core/web_contents.py
+++ b/tools/telemetry/telemetry/core/web_contents.py
@@ -82,6 +82,10 @@ class WebContents(object):
   def StartTimelineRecording(self, options=None):
     self._inspector_backend.StartTimelineRecording(options)
 
+  @property
+  def is_timeline_recording_running(self):
+    return self._inspector_backend.is_timeline_recording_running
+
   def StopTimelineRecording(self):
     self._inspector_backend.StopTimelineRecording()