Form validation: Do not show validation bubble if the page is invisible.

BUG=673163

Review-Url: https://codereview.chromium.org/2572813003
Cr-Commit-Position: refs/heads/master@{#438476}
void HTMLFormControlElement::requiredAttributeChanged() {
  setNeedsValidityCheck();
  pseudoStateChanged(CSSSelector::PseudoRequired);
  pseudoStateChanged(CSSSelector::PseudoOptional);
}

void HTMLFormControlElement::requiredAttributeChanged() {
  setNeedsValidityCheck();
  pseudoStateChanged(CSSSelector::PseudoRequired);
  pseudoStateChanged(CSSSelector::PseudoOptional);
}
