Now a descriptor array tracks the number of descriptors that were
already marked. The marking visitor of a map only marks the subset
of the descriptors that it needs and that are not already marked.

If a descriptor array is shared between M maps and has N descriptos,
then the number of marking operations is reduced from O(M*N) to O(N).

This patch also adds a marking barrier for descriptors.

The marked descriptor counter in a descriptor array is not cleared
after mark-compact GC. Instead, it embeds two bits from the global
mark-compact epoch counter and is considered 0 if the bits do not match
the current value of the global epoch counter.

Bug: v8:8486
Change-Id: I2a7822a6833f3143e1d351e5e4819c2ef2c07fb0
Reviewed-on: https://chromium-review.googlesource.com/c/1382746
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58372}