Make string test resilient against GC stress.
Make string test resilient against GC stress.

R=dcarney@chromium.org
TEST=cctest/test-strings/StringCharacterStreamRandom

Review URL: https://codereview.chromium.org/11565031

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@13220 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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