Switch to equalIgnoringASCIICase throughout modules/accessibility

BUG=627682

Review-Url: https://codereview.chromium.org/2793913007
Cr-Commit-Position: refs/heads/master@{#461858}
void AXLayoutObject::ariaLabelledbyElements(AXObjectVector& labelledby) const {
  accessibilityChildrenFromAttribute(aria_labelledbyAttr, labelledby);
}

void AXLayoutObject::ariaLabelledbyElements(AXObjectVector& labelledby) const {
  accessibilityChildrenFromAttribute(aria_labelledbyAttr, labelledby);
}
