[File Download Access Prevention] Add helper obfuscation methods

Initial simple functions for obfuscating and deobfuscating data using crypto::Aead which will be used to make the file contents inaccessible while the deep scan verdict is not received yet.

The key is generated randomly and reused for all downloads throughout the chrome lifetime. This might change later on as we might add key rotation, storage and derivation features in the future, but this feature acts as a deterrent for enterprise users and does not need to be secure for our current use case.

Will have follow-up CL for adding support for variable chunk sizes by prepending the current chunk's size as metadata to the encrypted chunk.

For more info see design doc go/cbe-dpr-download-access-prevention-dd.

Bug: b/349858060
Change-Id: I6b667457985fa788c575ff143d8808332ccc90d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5660244
Commit-Queue: Haihan Chen <haihan@google.com>
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1335444}