Call HeapObjectHeader::checkHeader solely for its side-effect.

This requires changing its signature. This is a preliminary stage to making it
private.

BUG=633030

Review-Url: https://codereview.chromium.org/2698673003
Cr-Commit-Position: refs/heads/master@{#460489}
int FreeList::bucketIndexForSize(size_t size) {
  ASSERT(size > 0);
  int index = -1;
  while (size) {
    size >>= 1;
    index++;
  }
  return index;
}

int FreeList::bucketIndexForSize(size_t size) {
  ASSERT(size > 0);
  int index = -1;
  while (size) {
    size >>= 1;
    index++;
  }
  return index;
}
