The Inspector will have an <input> element to execute user-provided JS.
This adds an IDL method and IPC to forward that JS from the Inspector
WebView to the Inspector client.
This is an internal object that must be explicitly enabled by the chrome
before it is added to the Window. The Inspector object will be used by a
special WebView that will replace all chrome-specific inspector windows.
The IDL defines methods that this WebView will need to inform the chrome
of various events, such as the user clicking a DOM node.
