This new class provides a unified interface for recording and iterating slots in store and slots buffers:

RememberedSet<OLD_TO_NEW>::Insert(page, slot);
RememberedSet<OLD_TO_OLD>::Insert(page, slot);

RememberedSet<OLD_TO_NEW>::Iterate(heap, callback);
RememberedSet<OLD_TO_OLD>::Iterate(heap, callback);

After this change the store buffer is responsible only for collecting slots from the generated code.

Subsequent CLs will remove the slots buffer.

BUG=chromium:578883
LOG=NO

Review URL: https://codereview.chromium.org/1683653002

Cr-Commit-Position: refs/heads/master@{#34031}