[oilpan] Fix GCInfoTable for multiple threads

Previously, grow and access from different threads could lead to a race
on the table backing; see bug.

- Rework the table to work on an existing reservation.
- Commit upon growing, avoiding any copies.

Drive-by: Fix over-allocation of table.

Bug: chromium:841280
Change-Id: I329cb6f40091e14e8c05334ba1104a9440c31d43
Reviewed-on: https://chromium-review.googlesource.com/1061525
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560434}
bool NormalPage::IsEmpty() {
  HeapObjectHeader* header = reinterpret_cast<HeapObjectHeader*>(Payload());
  return header->IsFree() && header->size() == PayloadSize();
}

bool NormalPage::IsEmpty() {
  HeapObjectHeader* header = reinterpret_cast<HeapObjectHeader*>(Payload());
  return header->IsFree() && header->size() == PayloadSize();
}
