Add HexDecode().

This is prework for merging
https://quiche.googlesource.com/quiche/+/9de6abe99a367775c3bfd3d001bda804cb1925a5,
which removes the transitively included Abseil header for
HexStringToBytes().  Abseil must not be used outside
net/*/platform/impl, to this CL changes usages to a helper function that
is defined in terms of base::HexStringToString().

Change-Id: I1585515a2c3c987ea176bba4461f33b61c9aa615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2856440
Auto-Submit: Bence Béky <bnc@chromium.org>
Commit-Queue: Fan Yang <fayang@chromium.org>
Reviewed-by: Fan Yang <fayang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#877450}
NOKEYCHECK=True
GitOrigin-RevId: bb49f0c3a0fdff459af86ddb2493aca402e357c5
7 files changed