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/extensions/chrome_api_registration.h

@@ -13,6 +13,9 @@ namespace extensions {
 namespace api {
 namespace cef {
 
+// Array of currently supported APIs.
+extern const char* const kSupportedAPIs[];
+
 class ChromeFunctionRegistry {
  public:
   static bool IsSupported(const std::string& name);