PartitionAlloc: add a discardSystemPages() hint API.

Source/wtf/PageAllocator.h

Index: Source/wtf/PageAllocator.h
diff --git a/Source/wtf/PageAllocator.h b/Source/wtf/PageAllocator.h
index bcde4e24a44c92d8f41a1ed4b39ae4e9e0654f77..572acc96f98758f5d46962c5150d7456e58f6489 100644
--- a/Source/wtf/PageAllocator.h
+++ b/Source/wtf/PageAllocator.h
@@ -107,6 +107,22 @@ WTF_EXPORT void decommitSystemPages(void* addr, size_t len);
 // len must be a multiple of kSystemPageSize bytes.
 WTF_EXPORT void recommitSystemPages(void* addr, size_t len);
 
+// Discard one or more system pages. Discarding is a hint to the system that
+// the page is no longer required. The hint may:
+// - Do nothing.
+// - Discard the page immediately, freeing up physical pages.
+// - Discard the page at some time in the future in response to memory pressure.
+// Only committed pages should be discarded. Discarding a page does not
+// decommit it, and it is valid to discard an already-discarded page.
+// A read or write to a discarded page will not fault.
+// Reading from a discarded page may return the original page content, or a
+// page full of zeroes.
+// Writing to a discarded page is the only guaranteed way to tell the system
+// that the page is required again. Once written to, the content of the page is // guaranteed stable once more. After being written to, the page content may be
+// based on the original page content, or a page of zeroes.
+// len must be a multiple of kSystemPageSize bytes.
+WTF_EXPORT void discardSystemPages(void* addr, size_t len);
+
 } // namespace WTF
 
 #endif // WTF_PageAllocator_h