Fix crash during GC in gen_snapshot:

Fix crash during GC in gen_snapshot:
- There were some uninitialized fields in object_store which could contain some
  random initial values. These fields are part of the root set used during
  Garbage collection and if a Garbage collection was triggered before these
  fields are formally initialized in Object::init, it could lead to random
  behaviour.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40794

[siva, 2014-09-29 22:35:11]

asiva@google.com changed reviewers:
+ regis@google.com

[regis, 2014-09-29 22:40:56]

LGTM!!

[siva, 2014-09-29 22:59:11]

Committed patchset #1 (id:1) manually as r40794 (presubmit successful).