gentlemen's room vs men's room

gentlemen's room

noun
  • Any room intended for use by men 

  • A waiting room in a railway station or other public building, often including a separate lavatory. 

men's room

noun
  • Other rooms intended for use by men, as waiting rooms, dressing rooms, locker rooms, etc. 

  • A lavatory intended for use by men, often including urinals in addition to toilets. 

How often have the words gentlemen's room and men's room occurred in a corpus of books? (source: Google Ngram Viewer )