Preparing the young generation for (real) non-contiguous backing memory, this
change removes object masks that are used to compute containment in semi and new
space. The masks are replaced by lookups for object tags and page headers, where
possible.

Details:
- Use the fast checks (page header lookups) for containment in regular code.
- Use the slow version that masks out the page start adress and iterates all
  pages of a space for debugging/verification.
- The slow version works for off-heap/unmapped memory.
- Encapsulate all checks for the old->new barrier in Heap::RecordWrite().

BUG=chromium:581412
LOG=N

Review URL: https://codereview.chromium.org/1632913003

Cr-Commit-Position: refs/heads/master@{#33857}