Add cleanup for measurements that use tracing

tools/telemetry/telemetry/page/page_measurement_unittest_base.py

Index: tools/telemetry/telemetry/page/page_measurement_unittest_base.py
diff --git a/tools/telemetry/telemetry/page/page_measurement_unittest_base.py b/tools/telemetry/telemetry/page/page_measurement_unittest_base.py
index 35c3db806ca6e6877fc6651d1f9168357acf58a6..b1e70ebb1c6c74f072c606fd27c6f42175abccae 100644
--- a/tools/telemetry/telemetry/page/page_measurement_unittest_base.py
+++ b/tools/telemetry/telemetry/page/page_measurement_unittest_base.py
@@ -51,3 +51,35 @@ class PageMeasurementUnitTestBase(unittest.TestCase):
     page_runner.ProcessCommandLineArgs(temp_parser, options)
     measurement.ProcessCommandLineArgs(temp_parser, options)
     return page_runner.Run(measurement, ps, expectations, options)
+
+  def TestTracingCleanedUp(self, measurement_class, options=None):
+    ps = self.CreatePageSetFromFileInUnittestDataDir('blank.html')
+    start_tracing_called = [False]
+    tracing_closed_called = [False]
+
+    class BuggyMeasurement(measurement_class):
+      def __init__(self, *args, **kwargs):
+        measurement_class.__init__(self, *args, **kwargs)
+
+      # Inject fake tracing methods to browser
+      def TabForPage(self, page, browser):
+        ActualStartTracing = browser.StartTracing
+        def FakeStartTracing(*args, **kwargs):
+          ActualStartTracing(*args, **kwargs)
+          start_tracing_called[0] = True
+          raise Exception('Intentional exception')
+        browser.StartTracing = FakeStartTracing
+
+        ActualStopTracing = browser.StopTracing
+        def FakeStopTracing(*args, **kwargs):
+          ActualStopTracing(*args, **kwargs)
+          tracing_closed_called[0] = True
+        browser.StopTracing = FakeStopTracing
+
+        return measurement_class.TabForPage(self, page, browser)
+
+    measurement = BuggyMeasurement()
+    self.RunMeasurement(measurement, ps, options=options)
+    assert start_tracing_called[0], ("%s measurement doesn't start tracing" %
+                                     measurement_class)
+    self.assertTrue(tracing_closed_called[0])