Add non-null terminating variant of modp_b64_encode

Some primitives in Blink (e.g. StringBuffer) require an O(n) allocation
if we shrink their size even by 1 character. This would mean adopting
modp_b64 in Blink code would cause a perf regression.

For now, this CL simply adds a variant which does not add the null
terminator.

Bug: 1383507
Change-Id: I405bc231db7155d343b1c42dda74e8189d6d1546
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4027430
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1074948}
NOKEYCHECK=True
GitOrigin-RevId: 39d5332257b3e5ef44f15ad0d9a5fa43bf8cc240

• README.chromium[diff]
• modp_b64.cc[diff]
• modp_b64.h[diff]