Remove tracing's nesting logic since noone is using it.

tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
index 218b95c5734a11551c177abebe4a350a33245033..5b20c3e9ac0d6233691657ded436eff840ed3b73 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/tracing_backend.py
@@ -25,9 +25,8 @@ class TracingBackend(object):
 
     self._inspector_websocket.Connect(
         'ws://127.0.0.1:%i/devtools/browser' % devtools_port)
-    self._category_filter = None
-    self._nesting = 0
     self._tracing_data = []
+    self._category_filter = None
     self._is_tracing_running = False
     self._chrome_browser_backend = chrome_browser_backend
 
@@ -36,10 +35,9 @@ class TracingBackend(object):
     return self._is_tracing_running
 
   def StartTracing(self, trace_options, custom_categories=None, timeout=10):
-    """ Starts tracing on the first nested call and returns True. Returns False
+    """ Starts tracing on the first call and returns True. Returns False
         and does nothing on subsequent nested calls.
     """
-    self._nesting += 1
     if self.is_tracing_running:

[dtu, 2014/09/15 22:06:33]

Maybe we don't need this check and/or this variable?
If we remove this we can also remove _category_filter?

[nednguyen, 2014/09/15 22:09:47]

No, if we allow StartTracing to be called multiple times. Or else, this need to
raise Exception to let caller knows that tracing is already running.

[chrishenry, 2014/09/23 17:49:26]

Should we throw exception? The return value from StopTracing is a bit weird if
it's nested.

[nednguyen, 2014/10/03 17:13:47]

done.

[nednguyen, 2014/10/03 17:13:47]

tracing_controller_backend return False in case it's started. Let's make it
consistent.

       new_category_filter = tracing_category_filter.TracingCategoryFilter(
           filter_string=custom_categories)
@@ -76,12 +74,9 @@ class TracingBackend(object):
     return True
 
   def StopTracing(self, timeout=30):
-    """ Stops tracing on the innermost (!) nested call, because we cannot get
-        results otherwise. Resets _tracing_data on the outermost nested call.
-        Returns the result of the trace, as TracingTimelineData object.
+    """ Stops tracing and returns the raw json trace result. It this is called
+    after tracing has been stopped, empty trace data is returned.
     """
-    self._nesting -= 1
-    assert self._nesting >= 0
     if self.is_tracing_running:
       req = {'method': 'Tracing.end'}
       self._inspector_websocket.SendAndIgnoreResponse(req)
@@ -98,19 +93,10 @@ class TracingBackend(object):
             'seconds. If the trace data is big, you may want to increase the '
             'time out amount.' % err.elapsed_time)
       self._is_tracing_running = False
-    if self._nesting == 0:
-      self._category_filter = None
-      return self._GetTraceResultAndReset()
-    else:
-      return self._GetTraceResult()
-
-  def _GetTraceResult(self):
-    assert not self.is_tracing_running
-    return self._tracing_data
+    return self._GetTraceResultAndReset()
 
   def _GetTraceResultAndReset(self):
-    result = self._GetTraceResult()
-
+    result = self._tracing_data
     self._tracing_data = []
     return result