Fix saving files encoded in base64
Fix saving files encoded in base64

A follow-up to https://crrev.com/c/1230042.

Change-Id: I7b786f41db42332c5addbd40b5a61a90e2764694
Reviewed-on: https://chromium-review.googlesource.com/c/1283750
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Ingvar Stepanyan <604d57d65e8bfa56d804ce9639e87f7f148ac601@cloudflare.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#600441}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4076ec268d128cf61f048d6069c0235db9e69e6c
ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend