SetVLOGLevel does not work if VLOG_IS_ON has already cached its module level

below is the minimal example that demonstrates the issue. 
I can send the possible fix.

```
int TestVlogHelper() {
  if (VLOG_IS_ON(1)) {
    return 1;
  }
  return 0;
}

void TestVLogModule() {
  int c = TestVlogHelper();
  CHECK_EQ(0, c);

  CHECK_EQ(0, SetVLOGLevel("logging_unittest", 1));
  c = TestVlogHelper();   
  CHECK_EQ(1, c);   // Fails here.
}
```