Added call to GetDC to check if Win32k enabled.

This CL adds a call to a Win32k API to check if it is really disabled
by the mitigation.

Change-Id: Ibab06e93db59c326b0df79ea562198e055beb88a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764178Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: James Forshaw <forshaw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689179}

sandbox/win/src/process_mitigations_unittest.cc

@@ -229,7 +229,11 @@ SBOX_TESTS_COMMAND int CheckPolicy(int argc, wchar_t** argv) {
         return SBOX_TEST_NOT_FOUND;
       }
       if (!policy.DisallowWin32kSystemCalls)
-        return SBOX_TEST_FAILED;
+        return SBOX_TEST_FIRST_ERROR;
+
+      // Check if we can call a Win32k API. Fail if it succeeds.
+      if (::GetDC(nullptr))
+        return SBOX_TEST_SECOND_ERROR;
 
       break;
     }