Update Chromium Rust policy (#2530)

Co-authored-by: Dustin J. Mitchell <[email protected]>

Author: adetaylor

src/chromium/policy.md

@@ -1,16 +1,29 @@
 # Chromium Rust policy
 
-Chromium does not yet allow first-party Rust except in rare cases as approved by
-Chromium's
-[Area Tech Leads](https://source.chromium.org/chromium/chromium/src/+/main:ATL_OWNERS).
+Chromium's Rust policy can be found
+[here](https://source.chromium.org/chromium/chromium/src/+/main:docs/rust.md;l=22).
+Rust can be used for both first-party and third-party code.
 
-Chromium's policy on third party libraries is outlined
-[here](https://chromium.googlesource.com/chromium/src/+/main/docs/adding_to_third_party.md#rust) -
-Rust is allowed for third party libraries under various circumstances, including
-if they're the best option for performance or for security.
+Using Rust for pure first-party code looks like this:
 
-Very few Rust libraries directly expose a C/C++ API, so that means that nearly
-all such libraries will require a small amount of first-party glue code.
+```bob
+"C++"                           Rust
+.- - - - - - - - - -.           .- - - - - - - - - - -.
+:                   :           :                     :
+: Existing Chromium :           :  Chromium Rust      :
+: "C++"             :           :  code               :
+: +---------------+ :           : +----------------+  :
+: |               | :           : |                |  :
+: |         o-----+-+-----------+-+->              |  :
+: |               | : Language  : |                |  :
+: +---------------+ : boundary  : +----------------+  :
+:                   :           :                     :
+`- - - - - - - - - -'           `- - - - - - - - - - -'
+```
+
+The third-party case is also common. It's likely that you'll also need a small
+amount of first-party glue code, because very few Rust libraries directly expose
+a C/C++ API.
 
 ```bob
 "C++"                           Rust
@@ -27,12 +40,9 @@ all such libraries will require a small amount of first-party glue code.
 `- - - - - - - - - -'           `- - - - - - - - - - - - - - - - - - - - - - -'
 ```
 
-> First-party Rust glue code for a particular third-party crate should normally
-> be kept in `third_party/rust/<crate>/<version>/wrapper`.
-
-Because of this, today's course will be heavily focused on:
+The scenario of using a third-party crate is the more complex one, so today's
+course will focus on:
 
 - Bringing in third-party Rust libraries ("crates")
-- Writing glue code to be able to use those crates from Chromium C++.
-
-If this policy changes over time, the course will evolve to keep up.
+- Writing glue code to be able to use those crates from Chromium C++. (The same
+  techniques are used when working with first-party Rust code).