Synchronous XHRs should be flagged in the resources Time graph to illustrate they are blocking script execution and preventing user interaction. This is related to bug 14351 which is about showing JS execution in the Time graph.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.