This is required for isolation (e.g. BeginFrameControl, focus) and simplifies the implementation of DevTools Browser domain commands for window sizes.

Current state:

on aura: WebContents created by HeadlessWebContents::Builder have their own WindowTreeHosts. Unclear what happens to WebContents created by the web content, e.g. via window.open(). We should ensure they also get their own WindowTreeHosts.

on mac: content layer creates NSView for each WebContents, unclear whether they belong to same NSWindow. We should ensure that, too.