Chrome Version: 72.3617

https://cs.chromium.org/chromium/src/net/cookies/cookie_util.cc?l=255&rcl=6adbc9ea5bd75e6b72fe1bb15a96511b4a7743ee

Chrome has:

  // Normalize the year to expand abbreviated years to the full year.
  if (exploded.year >= 69 && exploded.year <= 99)
    exploded.year += 1900;
  if (exploded.year >= 0 && exploded.year <= 68)
    exploded.year += 2000;

https://tools.ietf.org/html/rfc6265#section-5.2.1 says:

   If the year-value is greater than or equal to 70 and less than or
       equal to 99, increment the year-value by 1900.

   If the year-value is greater than or equal to 0 and less than or
       equal to 69, increment the year-value by 2000.

Firefox seems to match the spec.
https://dxr.mozilla.org/mozilla-central/source/nsprpub/pr/src/misc/prtime.c?q=%2Bfunction%3A%22PR_ParseTimeStringToExplodedTime%28const+char+%2A%2C+PRBool%2C+PRExplodedTime+%2A%29%22&redirect_type=single#964