[heap] Count bits in markbit cell instead of using a table.

Iterating live objects using mark bits is memory bound and already pretty heavy
on the cache as we always need to look at the mark bits, the objects (payload),
and their maps.

This CL changes the object start computation from a table lookup, which was yet
another memory location, to a CPU-bound computation.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1478623003

Cr-Commit-Position: refs/heads/master@{#32291}