heap: Implement DiscardSystemPages for Fuchsia

Bug: v8:12050
Change-Id: I9fa5c0d38f277e363befbdaecaaedd4ee55bfb07
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3071208Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76160}

src/base/platform/platform-fuchsia.cc

@@ -127,7 +127,10 @@ bool OS::SetPermissions(void* address, size_t size, MemoryPermission access) {
 
 // static
 bool OS::DiscardSystemPages(void* address, size_t size) {
-  // TODO(hpayer): Does Fuchsia have madvise?
+  uint64_t address_int = reinterpret_cast<uint64_t>(address);
+  zx_status_t status = zx::vmar::root_self()->op_range(
+      ZX_VMO_OP_DECOMMIT, address_int, size, nullptr, 0);
+  DCHECK_EQ(ZX_OK, status);
   return true;
 }