Inspector: replace generic discardAgents logic with explicit call to discard a listener.

Source/core/inspector/InspectorDOMAgent.cpp

Index: Source/core/inspector/InspectorDOMAgent.cpp
diff --git a/Source/core/inspector/InspectorDOMAgent.cpp b/Source/core/inspector/InspectorDOMAgent.cpp
index 2d703efc95d9dd16918db5a7355a70db916c0284..c84fee3fd686e550d0aa77679e27f2fc8b090412 100644
--- a/Source/core/inspector/InspectorDOMAgent.cpp
+++ b/Source/core/inspector/InspectorDOMAgent.cpp
@@ -238,6 +238,8 @@ InspectorDOMAgent::InspectorDOMAgent(InstrumentingAgents* instrumentingAgents, I
 
 InspectorDOMAgent::~InspectorDOMAgent()
 {
+    if (m_domListener)
+        m_domListener->willDestroyDOMAgent();
     reset();
     ASSERT(!m_searchingForNode);
 }