Update contributing.md to clarify the new architecture addition policy

This CL updates the policy based on the discussion at chrome-eng-review@.
https://groups.google.com/a/google.com/g/chrome-eng-review/c/0NLPHKImn4U?hl=en

Bug: None
Change-Id: I0ae50945b0ece308c4588926b6338ecee00cfa4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3554224
Reviewed-by: Erik Staab <[email protected]>
Commit-Queue: Kentaro Hara <[email protected]>
Auto-Submit: Kentaro Hara <[email protected]>
Cr-Commit-Position: refs/heads/main@{#986275}
diff --git a/docs/contributing.md b/docs/contributing.md
index d0d21ff..f3f312a3 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -331,9 +331,14 @@
   find a product in the Chromium repositories that depends on that line of code
   or else the line of code should be removed.
 
-  Completely new additions to the project (for example, support for a new OS
-  or architecture, or a new top-level directory for a new sub-project) must
-  be approved by [//ENG_REVIEW_OWNERS](../ENG_REVIEW_OWNERS).
+  Completely new additions to the project (e.g., support for a new OS or
+  architecture, or a new top-level directory for a new sub-project) must be
+  approved by [email protected]. For long-term maintenance reasons,
+  we will accept only things that are used by the Chromium project and things
+  that do not increase the cost of maintaining Chromium's supported
+  architectures / platforms (e.g., adding one ifdef branch for an unsupported
+  architecture / platform is fine but introducing new abstractions in the
+  codebase is problematic).
 
 - **Code should only be moved to a central location (e.g., //base) when
   multiple consumers would benefit.** We should resist the temptation to