[Telemetry] Add tab API function GetJavascriptStats().

tools/telemetry/telemetry/core/chrome/inspector_backend.py

Index: tools/telemetry/telemetry/core/chrome/inspector_backend.py
diff --git a/tools/telemetry/telemetry/core/chrome/inspector_backend.py b/tools/telemetry/telemetry/core/chrome/inspector_backend.py
index 8a5f8b109626db87602ad599b574d918ff8ab880..ee5c06cf58a4c70bea2b2a2fd9329567155a8210 100644
--- a/tools/telemetry/telemetry/core/chrome/inspector_backend.py
+++ b/tools/telemetry/telemetry/core/chrome/inspector_backend.py
@@ -9,6 +9,7 @@ import sys
 from telemetry.core import util
 from telemetry.core import exceptions
 from telemetry.core.chrome import inspector_console
+from telemetry.core.chrome import inspector_memory
 from telemetry.core.chrome import inspector_page
 from telemetry.core.chrome import inspector_runtime
 from telemetry.core.chrome import inspector_timeline
@@ -30,6 +31,7 @@ class InspectorBackend(object):
     self._next_request_id = 0
 
     self._console = inspector_console.InspectorConsole(self)
+    self._memory = inspector_memory.InspectorMemory(self)
     self._page = inspector_page.InspectorPage(self)
     self._runtime = inspector_runtime.InspectorRuntime(self)
     self._timeline = inspector_timeline.InspectorTimeline(self)
@@ -152,6 +154,16 @@ class InspectorBackend(object):
   def message_output_stream(self, stream):  # pylint: disable=E0202
     self._console.message_output_stream = stream
 
+  # Memory public methods.
+
+  def GetDOMStats(self, timeout):
+    dom_counters = self._memory.GetDOMCounters(timeout)
+    return {
+      'document_count': dom_counters['documents'],
+      'node_count': dom_counters['nodes'],
+      'event_listener_count': dom_counters['jsEventListeners']
+    }
+
   # Page public methods.
 
   def PerformActionAndWaitForNavigate(self, action_function, timeout):