[vm, gc] Discount two pages when calculating if a heap is fragmented.
[vm, gc] Discount two pages when calculating if a heap is fragmented.

The most recently allocated data and code pages may be partially used without indicating fragmentation. Discount two pages when calculating the usage ratio to avoid pathological detection of "fragmentation" in small heaps.

Bug: https://github.com/dart-lang/sdk/issues/30978
Change-Id: Ib3168fbdd6393c98c8c1cf88937a345339fad196
Reviewed-on: https://dart-review.googlesource.com/34500
Reviewed-by: Erik Corry <b520ba1e7f871f909b0b4a9e1cf9a391adc0e954@google.com>

dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk