clovis: Add a method to slim a trace by dropping large objects.

tools/android/loading/loading_trace.py

 # Copyright (c) 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Represents the trace of a page load."""
 
 import json
 
 import devtools_monitor
 import page_track
@@ skipping 16 matching lines @@
       url: (str) URL that has been loaded
       metadata: (dict) Metadata associated with the load.
       page: (PageTrack) instance of PageTrack.
       request: (RequestTrack) instance of RequestTrack.
       tracing_track: (TracingTrack) instance of TracingTrack.
     """
     self.url = url
     self.metadata = metadata
     self.page_track = page
     self.request_track = request
-    self.tracing_track = tracing_track
+    self._tracing_track = tracing_track
+    self._tracing_json_str = None
 
   def ToJsonDict(self):
     """Returns a dictionary representing this instance."""
     result = {self._URL_KEY: self.url, self._METADATA_KEY: self.metadata,
               self._PAGE_KEY: self.page_track.ToJsonDict(),
               self._REQUEST_KEY: self.request_track.ToJsonDict(),
               self._TRACING_KEY: self.tracing_track.ToJsonDict()}
     return result
 
   def ToJsonFile(self, json_path):
@@ skipping 39 matching lines @@
       LoadingTrace instance.
     """
     page = page_track.PageTrack(connection)
     request = request_track.RequestTrack(connection)
     trace = tracing.TracingTrack(
         connection,
         categories=(tracing.DEFAULT_CATEGORIES if categories is None
                     else categories))
     connection.MonitorUrl(url, timeout_seconds=timeout_seconds)
     return cls(url, chrome_metadata, page, request, trace)
+
+  @property
+  def tracing_track(self):
+    if not self._tracing_track:
+      self._RestoreTracingTrack()
+    return self._tracing_track
+
+  def Slim(self):
+    """Slims the memory usage of a trace by dropping the TraceEvents from it.
+
+    The tracing track is restored on-demand when accessed.
+    """
+    self._tracing_json_str = json.dumps(self._tracing_track.ToJsonDict())
+    self._tracing_track = None
+
+  def _RestoreTracingTrack(self):
+    assert self._tracing_json_str
+    self._tracing_track = tracing.TracingTrack.FromJsonDict(
+        json.loads(self._tracing_json_str))
+    self._tracing_json_str = None