Fixes a segfault that occurs when v8::CpuProfilers are restarted caused
by the reuse of a stale CodeEventObserver.

Bug: chromium:929928
Change-Id: I5d5f7eaf5cd903910130cdb0cfec8c3fd6608edd
Reviewed-on: https://chromium-review.googlesource.com/c/1459740Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59506}