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

Source/core/inspector/InspectorCSSAgent.cpp

Index: Source/core/inspector/InspectorCSSAgent.cpp
diff --git a/Source/core/inspector/InspectorCSSAgent.cpp b/Source/core/inspector/InspectorCSSAgent.cpp
index abd9e0bc195420327f3e142ffd68fc08797f2bbe..af1c57a7930c12ca817972426e2227d662584696 100644
--- a/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/Source/core/inspector/InspectorCSSAgent.cpp
@@ -695,7 +695,8 @@ InspectorCSSAgent::InspectorCSSAgent(InstrumentingAgents* instrumentingAgents, I
 
 InspectorCSSAgent::~InspectorCSSAgent()
 {
-    ASSERT(!m_domAgent);
+    if (m_domAgent)
+        m_domAgent->setDOMListener(0);
     reset();
 }
 
@@ -713,12 +714,6 @@ void InspectorCSSAgent::clearFrontend()
     stopSelectorProfilerImpl(0, false);
 }
 
-void InspectorCSSAgent::discardAgent()
-{
-    m_domAgent->setDOMListener(0);
-    m_domAgent = 0;
-}
-
 void InspectorCSSAgent::restore()
 {
     if (m_state->getBoolean(CSSAgentState::cssAgentEnabled)) {
@@ -1552,6 +1547,11 @@ void InspectorCSSAgent::didModifyDOMAttr(Element* element)
     it->value->didModifyElementAttribute();
 }
 
+void InspectorCSSAgent::willDestroyDOMAgent()
+{
+    m_domAgent = 0;
+}
+
 void InspectorCSSAgent::styleSheetChanged(InspectorStyleSheet* styleSheet)
 {
     if (m_frontend)