sandwich: Refactor to use more existing code.

tools/android/loading/tracing.py

Index: tools/android/loading/tracing.py
diff --git a/tools/android/loading/tracing.py b/tools/android/loading/tracing.py
index cf79908608e418e0e43be2f7511ebdf82c6ebf81..54ff3d08d4b6cfa87a4da1c27307d432d0c97822 100644
--- a/tools/android/loading/tracing.py
+++ b/tools/android/loading/tracing.py
@@ -12,12 +12,17 @@ import operator
 import devtools_monitor
 
 
+DEFAULT_CATEGORIES = None
+
+
 class TracingTrack(devtools_monitor.Track):
   """Grabs and processes trace event messages.
 
   See https://goo.gl/Qabkqk for details on the protocol.
   """
-  def __init__(self, connection, categories=None, fetch_stream=False):
+  def __init__(self, connection,
+               categories=DEFAULT_CATEGORIES,
+               fetch_stream=False):
     """Initialize this TracingTrack.
 
     Args:
@@ -304,6 +309,14 @@ class Event(object):
     return self._tracing_event['ph']
 
   @property
+  def category(self):
+    return self._tracing_event['cat']
+
+  @property
+  def pid(self):
+    return self._tracing_event['pid']
+
+  @property
   def args(self):
     return self._tracing_event.get('args', {})