Add flags to force or prevent setting of isolate.is_memory_constrained.

Also enable MAYBE_BOOL flags for when you want to only do something if the flag was explicitly set to true or false.

BUG=None
R=hpayer@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23890027

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16733 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
HeapSnapshot: replace O(N*ln(N)) algorithm of sorting with O(N) one.

We have HashMap for the strings. They got id sequentially. So we could use index sort.

BUG=none
R=alph@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/24174002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16734 ce2b1a6d-e550-0410-aec6-3dcde31c8c00