code/Debug/controllers/UtilController.php
public function enableFPCDebugAction()
    {
        try {
            $this->getService()->setFPCDebug(1);
            $this->getService()->flushCache();

            $message = $this->__('FPC debug was enabled');
            $this->getSession()->addSuccess($message);
        } catch (Exception $e) {
            $message = $this->__('FPC debug cannot be enabled: %s', $e->getMessage());
            $this->getSession()->addError($message);
        }

        $this->_redirectReferer();
    }
public function enableFPCDebugAction()
    {
        try {
            $this->getService()->setFPCDebug(1);
            $this->getService()->flushCache();

            $message = $this->__('FPC debug was enabled');
            $this->getSession()->addSuccess($message);
        } catch (Exception $e) {
            $message = $this->__('FPC debug cannot be enabled: %s', $e->getMessage());
            $this->getSession()->addError($message);
        }

        $this->_redirectReferer();
    }
Enables Full Page Cache Debug