Fix bug introduced by r21269.

Fix bug introduced by r21269.

PageSpace::LargePageSizeFor should not use sizeof(HeapPage) directly because real object start offset is additionally aligned.

BUG=

Committed: https://code.google.com/p/dart/source/detail?r=21276

[Vyacheslav Egorov (Google), 2013-04-11 12:59:05]

Please take a look.

We should also consider moving LargePageSizeFor into the HeapPage itself because
logically it belongs there.

[Vyacheslav Egorov (Google), 2013-04-11 13:06:47]

TBRing to extinguish the fire

[Vyacheslav Egorov (Google), 2013-04-11 13:08:22]

Committed patchset #1 manually as r21276 (presubmit successful).

[Vyacheslav Egorov (Google), 2013-04-11 13:11:15]

note: there is another use of sizeof(HeapPage)

  static const intptr_t kAllocatablePageSize = kPageSize - sizeof(HeapPage);

but it is safe.

I suggest we move all page size computations to the HeapPage itself.