Prepare for non-executable VMOs

Fuchsia VMOs will be non-executable by default in the future (SEC-42),
so we'll need to use zx_vmo_replace_as_executable to make it possible
to map them into memory as executable. And in the mean time, this call
is a no-op.

SEC-42
TBR: mlippautz

Change-Id: I4521559b163519ded4afa6b55e99f983e38b1de5
Reviewed-on: https://chromium-review.googlesource.com/c/1281824Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58804}

src/base/platform/platform-fuchsia.cc

@@ -57,6 +57,11 @@ void* OS::Allocate(void* address, size_t size, size_t alignment,
                          strlen(kVirtualMemoryName));
   uintptr_t reservation;
   uint32_t prot = GetProtectionFromMemoryPermission(access);
+  if ((prot & ZX_VM_FLAG_PERM_EXECUTE) != 0) {
+    if (zx_vmo_replace_as_executable(vmo, ZX_HANDLE_INVALID, &vmo) != ZX_OK) {
+      return nullptr;
+    }
+  }
   zx_status_t status = zx_vmar_map(zx_vmar_root_self(), prot, 0, vmo, 0,
                                    request_size, &reservation);
   // Either the vmo is now referenced by the vmar, or we failed and are bailing,