Don't break external links when Google Analytics is blocked or not loaded

This is a fix for https://crosswalk-project.org/jira/browse/XWALK-879

The fix is based on https://www.domsammut.com/code/workaround-for-when-the-hitcallback-function-does-not-receive-a-response-analytics-js

I changed it to `preventDefault()` and rely on `hitCallback` only when GA is actually loaded.