Bug 1604589: Fix default param in AutoVirtualProtect constructor; r=mhowell

We should be using `GetCurrentProcess` as the default, not `nullptr`.
`VirtualProtectEx` fails with a `nullptr` process handle.

Differential Revision: https://phabricator.services.mozilla.com/D57512

--HG--
extra : moz-landing-system : lando

widget/windows/WinHeaderOnlyUtils.h

@@ -457,7 +457,7 @@ class FileUniqueId final {
 class MOZ_RAII AutoVirtualProtect final {
  public:
   AutoVirtualProtect(void* aAddress, size_t aLength, DWORD aProtFlags,
-                     HANDLE aTargetProcess = nullptr)
+                     HANDLE aTargetProcess = ::GetCurrentProcess())
       : mAddress(aAddress),
         mLength(aLength),
         mTargetProcess(aTargetProcess),