Use phantom handle in GCObservation

The GCObservation handle is always set and empty state implies that the
observed object was garbage collection. Use phantom reference which
avoids clearing out the reference manually.

Bug: 903586, 843903
Change-Id: I77e6e9d9ef4625d4bb91b3907b0d1b69c17f2208
Reviewed-on: https://chromium-review.googlesource.com/c/1340320
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608771}
2 files changed