Any object the user can get access to via the debugger (as in, when at a breakpoint) should not be garbage collected out from under them.
We don't have any reason to think this can happen today. But it might in the future depending on how we design the Inspector IPC layer.
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.