Make cells pointing to JSObjects weak in optimized code.

This is done similar to weak embedded objects in optimized code (r17102). The
reference from optimized code to a cell is treated weakly in marking visitors
if the cell points to a JSObject. After marking we iterate over all cells
embedded in optimized code. If a cell is not marked but its value is marked,
then we revive the cell by marking it. Otherwise, the cell value is dead, so
we mark the code for deoptimization.

BUG=v8:2073
TEST=cctest/test-heap/CellsInOptimizedCodeAreWeak
LOG=Y
R=hpayer@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/117483002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18616 ce2b1a6d-e550-0410-aec6-3dcde31c8c00