Don't call StoreBuffer::ShutDown yet

Keep StoreBuffer::global_mutex_ and global_empty_ alive.

At Dart::CleanUp time, there may still be threads around that access their store buffer, so we need to keep these around until @zanderso lands the clean shutdown CL.

We should stop leaking them once we cleanly stop all threads at VM shutdown (see issue 23844).

BUG=https://github.com/dart-lang/sdk/issues/24169
R=iposva@google.com

Committed: https://github.com/dart-lang/sdk/commit/028c9952959be3b1f275c3cfbe805ab80c49f07a

[koda, 2015-08-21 19:58:59]

koda@google.com changed reviewers:
+ iposva@google.com, zra@google.com

[Ivan Posva, 2015-08-21 20:00:08]

LGTM -ip

[koda, 2015-08-21 20:08:01]

Per offline discussion, kept both the mutex and list, and just moved the
ShutDown call to the commented out section (which should be enabled soon).

[koda, 2015-08-21 20:09:20]

Committed patchset #2 (id:20001) manually as
028c9952959be3b1f275c3cfbe805ab80c49f07a (presubmit successful).