Replace CefRequestCallback with CefCallback (fixes issue #1861)

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

tests/ceftests/request_handler_unittest.cc

@@ -97,7 +97,7 @@ class NetNotifyTestHandler : public TestHandler {
       CefRefPtr<CefBrowser> browser,
       CefRefPtr<CefFrame> frame,
       CefRefPtr<CefRequest> request,
-      CefRefPtr<CefRequestCallback> callback) override {
+      CefRefPtr<CefCallback> callback) override {
     EXPECT_TRUE(CefCurrentlyOn(TID_IO));
 
     const std::string& url = request->GetURL();