Run V8 GC before detecting Document leaks in inspector_memory_unittest.py.

Previously, this unit test was consistently failing because DOM wrappers were
retained at the time of detecting Document leaks. This patch adds a call to
V8's gc() before the leak check to make sure the DOM wrappers are freed.

BUG=422244
R=kouhei@chromium.org

Review URL: https://codereview.chromium.org/882913002

Cr-Commit-Position: refs/heads/master@{#313471}

tools/telemetry/telemetry/core/backends/chrome_inspector/inspector_memory_unittest.py

@@ -9,7 +9,6 @@ from telemetry.unittest_util import tab_test_case
 class InspectorMemoryTest(tab_test_case.TabTestCase):
 
   @decorators.Enabled('has tabs')
-  @decorators.Disabled  # http://crbug.com/422244
   def testGetDOMStats(self):
     # Due to an issue with CrOS, we create a new tab here rather than
     # using the existing tab to get a consistent starting page on all platforms.
@@ -17,6 +16,8 @@ class InspectorMemoryTest(tab_test_case.TabTestCase):
 
     self.Navigate('dom_counter_sample.html')
 
+    self._tab.ExecuteJavaScript('gc();')
+
     # Document_count > 1 indicates that WebCore::Document loaded in Chrome
     # is leaking! The baseline should exactly match the numbers on:
     # unittest_data/dom_counter_sample.html
@@ -29,3 +30,7 @@ class InspectorMemoryTest(tab_test_case.TabTestCase):
         'Node leak is detected!')
     self.assertEqual(counts['event_listener_count'], 2,
         'EventListener leak is detected!')
+
+  @classmethod
+  def CustomizeBrowserOptions(cls, options):
+    options.AppendExtraBrowserArgs('--js-flags=--expose-gc')