If a script is unloaded between the collection of an allocation and the
tranlation of an allocation profile, the profiler will segfault. With
this change, we report unloaded scripts as having no line number,column
number, or name.

R=ofrobots@google.com
BUG=

Review URL: https://codereview.chromium.org/1846723002

Cr-Commit-Position: refs/heads/master@{#35147}