As part of the page type unification also unify page iterators. Iterating
over a space works the same for all spaces now (new, old, lo).

Iterating over pages of a space follows now the regular C++ iterator pattern:
- for (auto it = space->begin(); it != space->end(); ++it) {}
- for (Page* p : *space) {}

GC only: Loop supporting unlinking/freeing of a Page on the fly:
  for (auto it = space->begin(); != space->end();) {
    Page* p = *(it++);
    p->Unlink();
  }

For iteration of a range of new space pages use NewSpacePageRange which
also verifies that the range is actually a proper new space page range.

BUG=chromium:581412
LOG=N

Review-Url: https://codereview.chromium.org/2088223002
Cr-Commit-Position: refs/heads/master@{#37210}