LibWeb+Browser: Support DOM Inspector for OutOfProcessWebView

This introduces a new DOMTreeJSONModel, which provides the Model for the
InspectorWidget when the Browser is running using the
OutOfProcessWebView.

This Model is constructed with a JSON object received via IPC from the
WebContentServer.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -64,6 +64,7 @@ set(SOURCES
     DOM/Timer.cpp
     DOM/Window.cpp
     DOMTreeModel.cpp
+    DOMTreeJSONModel.cpp
     Dump.cpp
     FontCache.cpp
     HTML/AttributeNames.cpp