Bug 1734650 - Ensure that LoadLibrarySystem32 loads dependencies from System32. r=handyman

Use LOAD_LIBRARY_SEARCH_SYSTEM32 if available. Otherwise use
LOAD_WITH_ALTERED_SEARCH_PATH.

Removed an anonymous namespace to share static variables. This namespace
was necessary in order to avoid ODR violation before [this changeset](https://searchfox.org/mozilla-central/diff/416d586a1420dd82094c0cac417ee01213eb9ad6/xpcom/base/nsWindowsHelpers.h#160).
It is not required anymore.

Differential Revision: https://phabricator.services.mozilla.com/D158832

xpcom/base/nsWindowsHelpers.h

@@ -253,8 +253,6 @@ typedef nsAutoRef<nsHGLOBAL> nsAutoGlobalMem;
 typedef nsAutoRef<nsHPRINTER> nsAutoPrinter;
 typedef nsAutoRef<MSIHANDLE> nsAutoMsiHandle;
 
-namespace {
-
 // Construct a path "<system32>\<aModule>". return false if the output buffer
 // is too small.
 // Note: If the system path cannot be found, or doesn't fit in the output buffer
@@ -300,15 +298,18 @@ bool inline ConstructSystem32Path(LPCWSTR aModule, WCHAR* aSystemPath,
 }
 
 HMODULE inline LoadLibrarySystem32(LPCWSTR aModule) {
+  static const auto setDefaultDllDirectories =
+      GetProcAddress(GetModuleHandleW(L"kernel32"), "SetDefaultDllDirectories");
+  if (setDefaultDllDirectories) {
+    return LoadLibraryExW(aModule, nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32);
+  }
   WCHAR systemPath[MAX_PATH + 1];
   if (!ConstructSystem32Path(aModule, systemPath, MAX_PATH + 1)) {
     return NULL;
   }
-  return LoadLibraryW(systemPath);
+  return LoadLibraryExW(systemPath, nullptr, LOAD_WITH_ALTERED_SEARCH_PATH);
 }
 
-}  // namespace
-
 // for UniquePtr
 struct LocalFreeDeleter {
   void operator()(void* aPtr) { ::LocalFree(aPtr); }