Add initial WebUI support (issue #2037)

- Visit chrome://webui-hosts for the list of supported hosts.

libcef/browser/browser_context_proxy.h

@@ -23,6 +23,9 @@ class CefBrowserContextProxy : public CefBrowserContext {
   CefBrowserContextProxy(CefRefPtr<CefRequestContextHandler> handler,
                          scoped_refptr<CefBrowserContextImpl> parent);
 
+  // Must be called immediately after this object is created.
+  void Initialize() override;
+
   // SupportsUserData methods.
   Data* GetUserData(const void* key) const override;
   void SetUserData(const void* key, Data* data) override;