Fixing remaining VC++ 2015 64-bit build breaks

sandbox/win/src/policy_target_test.cc

Index: sandbox/win/src/policy_target_test.cc
diff --git a/sandbox/win/src/policy_target_test.cc b/sandbox/win/src/policy_target_test.cc
index bb1f0b2ba38fd2fb94c780c26715a42c74975e2d..e286073d743b0675b7636ea8c5d0e42e1dc74622 100644
--- a/sandbox/win/src/policy_target_test.cc
+++ b/sandbox/win/src/policy_target_test.cc
@@ -382,8 +382,10 @@ TEST(PolicyTargetTest, ShareHandleTest) {
   base::string16 arguments(L"\"");
   arguments += prog_name;
   arguments += L"\" -child 0 shared_memory_handle ";
+  // Cast through uintptr_t and then unsigned int to make the truncation
+  // explicit. Handles are size-of-pointer but are always 32-bit values.
   arguments += base::UintToString16(
-      reinterpret_cast<unsigned int>(shared_handle));
+      static_cast<unsigned int>(reinterpret_cast<uintptr_t>(shared_handle)));
 
   // Launch the app.
   ResultCode result = SBOX_ALL_OK;