Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/request_context_impl.h

@@ -80,6 +80,13 @@ class CefRequestContextImpl : public CefRequestContext {
   cef_errorcode_t ResolveHostCached(
       const CefString& origin,
       std::vector<CefString>& resolved_ips) override;
+  void LoadExtension(const CefString& root_directory,
+                     CefRefPtr<CefDictionaryValue> manifest,
+                     CefRefPtr<CefExtensionHandler> handler) override;
+  bool DidLoadExtension(const CefString& extension_id) override;
+  bool HasExtension(const CefString& extension_id) override;
+  bool GetExtensions(std::vector<CefString>& extension_ids) override;
+  CefRefPtr<CefExtension> GetExtension(const CefString& extension_id) override;
 
   const CefRequestContextSettings& settings() const { return config_.settings; }