Prepration for returning memory to the OS on Windows.  Track whether a portion of a span of memory was returned to the OS.
If it was, ask that it be recommitted before returning it to the application as an allocated region.

Reviewed by Oliver Hunt.

* wtf/FastMalloc.cpp:
(WTF::TCMalloc_PageHeap::New):  If the span was decommitted, ask that it be recommitted before returning it.
(WTF::TCMalloc_PageHeap::AllocLarge):  Ditto.
(WTF::TCMalloc_PageHeap::Carve):  When splitting a span, ensure that the decommitted state propogates to the two new spans.
(WTF::TCMalloc_PageHeap::Delete):  When merging a span, ensure that the resulting span is marked as decommitted if any of the
spans being merged were marked as decommitted.
(WTF::TCMalloc_PageHeap::IncrementalScavenge):  Mark as decommitted after releasing the span.
(WTF::TCMalloc_Central_FreeList::FetchFromSpans): Add an assertion to catch a decommitted span being returned to the application
without first being recommitted.
(WTF::TCMalloc_Central_FreeList::Populate): Ditto.
* wtf/TCSystemAlloc.cpp: Stub out TCMalloc_SystemCommit.
* wtf/TCSystemAlloc.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@34756 268f45cc-cd09-0410-ab3c-d52691b4dbfc