Use weak references on cache eviction.
Fix up crash around weak ref usage

Change-Id: If074d75e410a2db1391654d48069e5381ffa9823
Fix up crash around weak ref usage

Change-Id: I45198f1b3ba4ad5a657f323aa322f0ac3808d6d4
Merge branch 'master' of https://github.com/koush/ion

# By Koushik Dutta
# Via Koushik Dutta
* 'master' of https://github.com/koush/ion:
  Fix up crash around weak ref usage
  Fix up crash around weak ref usage
  Use weak references on cache eviction.