Calendar picker: Remove hard-coded Japanese era detection logic

Intl.DateTimeFormat can produce Japanese era names. calendarPicker.js just does
 - Drop eras older than Meiji
 - Drop 100+ era years
 - Replace 1-nen with gan-nen

No user-visible behavior changes except for the results for 1968
January-October.  Meiji has two beginning dates, and we applied
the former date, and Intl.DateTimeForma applies the latter date.

Change-Id: I6f3619c7280e4c18aff25fa16a160fc0e4ba1c4e
Bug: 919712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600951
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658043}