Make object-observe test case resilient against GC stress.
Make object-observe test case resilient against GC stress.

R=rossberg@chromium.org
TEST=cctest/test-object-observe/ObservationWeakMap

Review URL: https://codereview.chromium.org/12217017

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@13597 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh