LibWebView+WebContent: Create a different console client for DevTools

Our existing WebContentConsoleClient is very specific to our home-grown
Inspector. It renders console output to an HTML string. For DevTools, we
will not want this behavior; we will want to send representations of raw
JS values.

This patch makes WebContentConsoleClient a base class to handle console
input from the user, either from the Inspector or from DevTools. It then
moves the HTML rendering needed for the Inspector to a new class,
InspectorConsoleClient. And we add a DevToolsConsoleClient (currently
just stubbed) to handle needs specific to DevTools.

We choose at runtime which console client to install, based on the
--devtools command line flag.

Services/WebContent/Forward.h

@@ -10,6 +10,8 @@ namespace WebContent {
 
 class ConnectionFromClient;
 class ConsoleGlobalEnvironmentExtensions;
+class DevToolsConsoleClient;
+class InspectorConsoleClient;
 class PageHost;
 class PageClient;
 class WebContentConsoleClient;