DevTools: expose setLayerTreeId() so we can relate events from LayerTreeHost

Source/core/inspector/InspectorTimelineAgent.h

Index: Source/core/inspector/InspectorTimelineAgent.h
diff --git a/Source/core/inspector/InspectorTimelineAgent.h b/Source/core/inspector/InspectorTimelineAgent.h
index b58915660d617ba7e036a9b0966a78f52502de3b..574a4cbc0087421e9de9dc9420e8921d09755ac3 100644
--- a/Source/core/inspector/InspectorTimelineAgent.h
+++ b/Source/core/inspector/InspectorTimelineAgent.h
@@ -121,6 +121,8 @@ public:
     virtual void start(ErrorString*, const int* maxCallStackDepth, const bool* includeDomCounters, const bool* includeNativeMemoryStatistics);
     virtual void stop(ErrorString*);
 
+    void setLayerTreeId(int layerTreeId) { m_layerTreeId = layerTreeId; }
+    int layerTreeId() const { return m_layerTreeId; }
     int id() const { return m_id; }
 
     void didCommitLoad();
@@ -285,6 +287,7 @@ private:
     WeakPtrFactory<InspectorTimelineAgent> m_weakFactory;
     RefPtr<TimelineTraceEventProcessor> m_traceEventProcessor;
     unsigned m_styleRecalcElementCounter;
+    int m_layerTreeId;
 };
 
 } // namespace WebCore