boolean doubleBackToExitPressedOnce = false;

    @Override
    public void onBackPressed() {
        if(webView.canGoBack())
        {
            webView.goBack();
        }
        else
        {
            if (doubleBackToExitPressedOnce) {
                super.onBackPressed();
                return;
            }

            this.doubleBackToExitPressedOnce = true;
            Toast.makeText(this, "Çıkmak için Geri'ye bir kere daha basınız.", Toast.LENGTH_SHORT).show();

            new Handler().postDelayed(new Runnable() {

                @Override
                public void run() {
                    doubleBackToExitPressedOnce=false;
                }
            }, 2000);
        }
    }