To improve the performance of the DOM Inspector when the Browser is run
in multi-process mode, do not create copies of the JSON values sent via
IPC when searching for a model index. Methods that are guaranteed to
return a value now return a reference. Methods that do not have such a
guarantee return a pointer (rather than an Optional, because Optional
cannot hold references).
The DOM Inspector performs well at first, but will start lagging again
once the tree is expanded a few nodes deep and/or with many nodes
visible in the tree.
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.