Add warnings to inspector DOM count unittest baselines.

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

Index: tools/telemetry/telemetry/core/backends/chrome/inspector_memory_unittest.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/inspector_memory_unittest.py b/tools/telemetry/telemetry/core/backends/chrome/inspector_memory_unittest.py
index 710db05b1732157e3c4c7a9fa899ed028208e265..59d1ce898826927bf0b6034ffcd99855c8e1a64a 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/inspector_memory_unittest.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/inspector_memory_unittest.py
@@ -16,7 +16,15 @@ class InspectorMemoryTest(tab_test_case.TabTestCase):
 
     self.Navigate('dom_counter_sample.html')
 
+    # 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
+    # Please contact kouhei@, hajimehoshi@ when rebaselining.
     counts = self._tab.dom_stats
-    self.assertEqual(counts['document_count'], 1)
-    self.assertEqual(counts['node_count'], 14)
-    self.assertEqual(counts['event_listener_count'], 2)
+    self.assertEqual(counts['document_count'], 1,
+        'Document leak is detected! '+
+        'The previous document is likely retained unexpectedly.')
+    self.assertEqual(counts['node_count'], 14,
+        'Node leak is detected!')
+    self.assertEqual(counts['event_listener_count'], 2,
+        'EventListener leak is detected!')