Make incremental marking clear type feedback cells.

Make incremental marking clear type feedback cells.

This extends the existing clearing of type feedback cells during GC to
incremental marking in order to prevent cross-context retention that
would last until the next non-incremental GC.

R=erik.corry@gmail.com
TEST=cctest/test-heap/IncrementalMarkingClearsTypeFeedbackCells

Committed: https://code.google.com/p/v8/source/detail?r=12245

[Erik Corry, 2012-07-31 10:13:04]

LGTM