[reland] Enable *building* Rusty QR Code Generation on Android and Linux

This is a reland of the core part of https://crrev.com/c/4509543.  The
reland has been made possible by the following fixes:
- https://crrev.com/c/4636942 (split out from the original CL)
- https://crrev.com/c/4633412
- https://crrev.com/c/4634895

This CL enables *building* of Rusty QR Code Generation on the same
platforms and configs where `enable_rust_crash` is built.  It changes
both `enable_rust_qr` and `enable_rust_base_conversions`, because
QR-related FFI requires conversions from `base::span<T>` into
`rust::Slice<T>`.

This CL does *not* yet *use* Rusty QR Code Generation at runtime,
because the "RustyQrCodeGenerator" feature remains disabled by default.

Since this CL starts to include the `third_party/rust/qr_code` library,
it expectedly leads to a ~22kB binary size increase.  This increase has
been reviewed and approved when the ATLs have reviewed landing of the
third-party library (see https://crrev.com/c/4497329 and the approvals
listed from that other CL's description).  Also note that evaluating the
binary size impact of shipping Rust code is tricky, because the cost of
the base C++ libraries is amortized across multiple components, while
the cost of the base Rust libraries will initially only support the QR
crate.

Bug: 1431991
Binary-Size: Size increase is unavoidable (see above).
Change-Id: I380d904f5370d5ee129179abd0a0b949474b53cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4632898
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1161494}