1. Avoid using SKIP_WRITE_BARRIER when we don't have to (smis).
  2. Check and document the remaining uses of SKIP_WRITE_BARRIER.
  3. Only allow GetWriteBarrierMode when in an AssertNoAllocation scope.

The only functional change should be in DeepCopyBoilerplate where we
no longer use the write barrier mode (because of allocations). I'm
running benchmarks to see if this has a measurable impact on performance.
Review URL: http://codereview.chromium.org/558041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3743 ce2b1a6d-e550-0410-aec6-3dcde31c8c00