Replace CefRequestCallback with CefCallback (fixes issue #1861)

CefRequestCallback::Continue(false) was the same as Cancel(), so we can just
use CefCallback instead.

tests/ceftests/scheme_handler_unittest.cc

@@ -144,7 +144,7 @@ class TestSchemeHandler : public TestHandler {
       CefRefPtr<CefBrowser> browser,
       CefRefPtr<CefFrame> frame,
       CefRefPtr<CefRequest> request,
-      CefRefPtr<CefRequestCallback> callback) override {
+      CefRefPtr<CefCallback> callback) override {
     if (IsChromeRuntimeEnabled() && request->GetResourceType() == RT_FAVICON) {
       // Ignore favicon requests.
       return RV_CANCEL;