Test more AddResult of HashMap::add() behavior
Test more AddResult of HashMap::add() behavior

Was wondering if I can use AddResult.storedValue to modify the
existing value when HashMap::add() is called with existing key
so that I can simply use:
  vectorMap.add(key, XVector()).storedValue->value.append(x)
instead of much longer form:
  VectorMap::iterator it = vectorMap.find(key);
  if (it == vectorMap.end()) {
    XVector v;
    v.append(x);
    vectorMap.add(key, v);
  } else {
    it->value.append(x);
  }

The new test verifies that we can use the simple form.

Review URL: https://codereview.chromium.org/959323002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@191125 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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