Simplify SignedExchange certificate's validity period check

Before this patch, SignedExchange certificates with validity periods
longer than 90 days were allowed if:

- current time is before 2019-08-01 00:00 UTC, and
- the certificate was issued before 2019-05-01.

Now the first condition is always false, so we only have to check the
validity period.

Bug: 953165
Change-Id: Iba4d8a4b067efce9acb698329be90a32c9a1728d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728578
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683067}
34 files changed