PartitionAlloc: add a discardSystemPages() hint API.

Source/wtf/PageAllocator.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 203 matching lines @@
 void recommitSystemPages(void* addr, size_t len)
 {
     ASSERT(!(len & kSystemPageOffsetMask));
 #if OS(POSIX)
     (void) addr;
 #else
     RELEASE_ASSERT(setSystemPagesAccessible(addr, len));
 #endif
 }
 
+void discardSystemPages(void* addr, size_t len)
+{
+    ASSERT(!(len & kSystemPageOffsetMask));
+#if OS(POSIX)
+    // On POSIX, the implementation detail is that discard and decommit are the
+    // same, and lead to pages that are returned to the system immediately and
+    // get replaced with zeroed pages when touched. So we just call
+    // decommitSystemPages() here to avoid code duplication.
+    decommitSystemPages(addr, len);
+#else
+    (void) addr;
+    (void) len;
+    // TODO(cevans): implement this using MEM_RESET for Windows, once we've
+    // decided that the semantics are a match.
+#endif
+}
+
 } // namespace WTF