Make HandleCloserAgent walk in 4x multiples on x64

sandbox/win/src/handle_closer_agent.cc

Index: sandbox/win/src/handle_closer_agent.cc
diff --git a/sandbox/win/src/handle_closer_agent.cc b/sandbox/win/src/handle_closer_agent.cc
index be0ffc17924a06e2a3756948f3aeab965d79f616..07c6a09854d2d5355a84f30f782ec57f1a04f3ab 100644
--- a/sandbox/win/src/handle_closer_agent.cc
+++ b/sandbox/win/src/handle_closer_agent.cc
@@ -78,7 +78,7 @@ void HandleCloserAgent::InitializeHandlesToClose() {
 bool HandleCloserAgent::CloseHandles() {
   DWORD handle_count = UINT_MAX;
   const int kInvalidHandleThreshold = 100;
-  const size_t kHandleOffset = sizeof(HANDLE);
+  const size_t kHandleOffset = 4;  // Handles are always a multiple of 4.
 
   if (!::GetProcessHandleCount(::GetCurrentProcess(), &handle_count))
     return false;