RFC: Try to be much more careful with where we skip the write barrier by:...

RFC: Try to be much more careful with where we skip the write barrier by:
  
  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.

Committed: http://code.google.com/p/v8/source/detail?r=3743

[Mads Ager (chromium), 2010-01-29 10:13:58]

This looks great!  Please commit if the boilerplate copying change is not a
performance issue. :)