If client releases BarrierClosure during |done_closure_| invocation, the closure
is destructed together with BarrierInfo object that it owns. After that
BarrierInfo tries to call |done_closure_.Reset()| which leads to SIGSEGV.
Fixed that by saving |done_clusure| to a stack variable.

BUG=none
TEST=base_unittests --gtest_filter=BarrierClosureTest.KeepingClosureAliveUntilDone

Review URL: https://codereview.chromium.org/1116833002

Cr-Commit-Position: refs/heads/master@{#328669}