Web Inspector: Adopt ES6 Class Syntax for all View Objects.

[Matt Baker]

* SUMMARY
Adopt ES6 Class Syntax for all View Objects. Some view constructors (I'm looking at you, ContentView) follow an implicit factory pattern whereby the base class constructor returns a concrete type based on the represented object. We should make the factory pattern explicit using a static class method:

class ContentView {
    static create(representedObject) {
        if (typeof representedObject === "string" || representedObject instanceof String)
            return new WebInspector.TextContentView(representedObject);

        // And so on...

        console.assert(!WebInspector.ContentView.isViewable(representedObject));
        throw "Can't make a ContentView for an unknown representedObject.";
    }
}

* NOTES
The const keyword isn't allowed in strict mode, and ES6 class method scope uses strict mode implicitly. Instances of const will need to be replaced with var.

[Radar WebKit Bug Importer]

<rdar://problem/20233935>

[Timothy Hatcher]

Created attachment 249448 [details]
WIP

[Timothy Hatcher]

Created attachment 249449 [details]
WIP (Ignore Space)

[Timothy Hatcher]

Created attachment 249557 [details]
WIP

Latest WIP after landing some of the working classes.