[Android] Add ability to generate test trace json for perf tests runs.

build/android/pylib/local/device/local_device_perf_test_run.py

Index: build/android/pylib/local/device/local_device_perf_test_run.py
diff --git a/build/android/pylib/local/device/local_device_perf_test_run.py b/build/android/pylib/local/device/local_device_perf_test_run.py
index efb00017c2eb61e1619a062266cf28d2ecf9e573..74ed5d595a444e623179cc29d29f649580313011 100644
--- a/build/android/pylib/local/device/local_device_perf_test_run.py
+++ b/build/android/pylib/local/device/local_device_perf_test_run.py
@@ -29,6 +29,7 @@ from pylib.base import base_test_result
 from pylib.constants import host_paths
 from pylib.local.device import local_device_environment
 from pylib.local.device import local_device_test_run
+from py_trace_event import trace_event
 
 
 class HeartBeat(object):
@@ -96,8 +97,12 @@ class TestShard(object):
 
     try:
       start_time = time.time()
+      if self._test_instance.trace_output:

[jbudorick, 2016/12/15 20:22:33]

My vote would be for a context manager.

In related news, I need to get around to writing a conditional context manager
or something like that in catapult/common/py_utils.

[jbudorick, 2016/12/15 20:22:59]

that and tempdir as a context manager.

[rnephew (Reviews Here), 2016/12/15 21:24:04]

Moved to context manager.

+        trace_event.trace_begin(test)
       exit_code, output = cmd_helper.GetCmdStatusAndOutputWithTimeout(
           cmd, timeout, cwd=cwd, shell=True)
+      if self._test_instance.trace_output:
+        trace_event.trace_end(test)
       end_time = time.time()
       json_output = self._test_instance.ReadChartjsonOutput(self._output_dir)
       if exit_code == 0:
@@ -410,6 +415,9 @@ class LocalDevicePerfTestRun(local_device_test_run.LocalDeviceTestRun):
   #override
   def RunTests(self):
     # Affinitize the tests.
+    if self._test_instance.trace_output:
+      assert not trace_event.trace_is_enabled(), 'Tracing already running.'
+      trace_event.trace_enable(self._test_instance.trace_output)
     self._SplitTestsByAffinity()
     if not self._test_buckets and not self._no_device_tests:
       raise local_device_test_run.NoTestsError()
@@ -447,6 +455,9 @@ class LocalDevicePerfTestRun(local_device_test_run.LocalDeviceTestRun):
 
     host_test_results, device_test_results = reraiser_thread.RunAsync(
         [run_no_devices_tests, run_devices_tests])
+    if self._test_instance.trace_output:
+      assert trace_event.trace_is_enabled(), 'Tracing not running.'
+      trace_event.trace_disable()
     return host_test_results + device_test_results
 
   # override