Do not base64 encode serialized PRT in manager

Serialized PRTs will be added to relevant header as a byte sequence
using structured headers. Serializing a byte sequence base64 encodes
the given bytes. Remove base64 encoding in PRT manager to prevent
double base64 encoding.

Bug: 402425762
Change-Id: I8088af5da01e4c0a1a6852633651e2a4d4d19f76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6438022
Reviewed-by: Ryan Kalla <ryankalla@google.com>
Reviewed-by: Dustin Mitchell <djmitche@chromium.org>
Commit-Queue: Aykut Bulut <aykutb@google.com>
Cr-Commit-Position: refs/heads/main@{#1444136}