[heap] Reland uncommit unused large object page memory.

src/base/platform/platform.h

Index: src/base/platform/platform.h
diff --git a/src/base/platform/platform.h b/src/base/platform/platform.h
index 9464fb11231dbcd59efed2846ff4d4894681f83f..d3b6c9c1cf3d758276ccc39c50a20b51fc9f9f98 100644
--- a/src/base/platform/platform.h
+++ b/src/base/platform/platform.h
@@ -337,6 +337,23 @@ class VirtualMemory {
   // Creates a single guard page at the given address.
   bool Guard(void* address);
 
+  // Releases the memory after |free_start|.
+  void ReleasePartial(void* free_start) {
+    DCHECK(IsReserved());
+    // Notice: Order is important here. The VirtualMemory object might live
+    // inside the allocated region.
+    size_t size = size_ - (reinterpret_cast<size_t>(free_start) -
+                           reinterpret_cast<size_t>(address_));
+    CHECK(InVM(free_start, size));
+    DCHECK_LT(address_, free_start);
+    DCHECK_LT(free_start, reinterpret_cast<void*>(
+                              reinterpret_cast<size_t>(address_) + size_));
+    bool result = ReleasePartialRegion(address_, size_, free_start, size);
+    USE(result);
+    DCHECK(result);
+    size_ -= size;
+  }
+
   void Release() {
     DCHECK(IsReserved());
     // Notice: Order is important here. The VirtualMemory object might live
@@ -369,6 +386,12 @@ class VirtualMemory {
   // and the same size it was reserved with.
   static bool ReleaseRegion(void* base, size_t size);
 
+  // Must be called with a base pointer that has been returned by ReserveRegion
+  // and the same size it was reserved with.
+  // [free_start, free_start + free_size] is the memory that will be released.
+  static bool ReleasePartialRegion(void* base, size_t size, void* free_start,
+                                   size_t free_size);
+
   // Returns true if OS performs lazy commits, i.e. the memory allocation call
   // defers actual physical memory allocation till the first memory access.
   // Otherwise returns false.