Update contributing doc to clarify how big new things get added.

The //docs/contributing.md file talks about how all code in
Chromium must be relevant to the project, but as written it was
somewhat circular ("should be in service of other code"); this
CL updates the wording a bit to try and clarify how completely new
top-level things (like //fuchsia) get approved.

R=jochen@chromium.org, haraken@chromium.org, darin@chromium.org

Change-Id: I35246fff40d3cec0700e55d904f63973db2eaee0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2246830Reviewed-by: Darin Fisher <darin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#779061}

docs/contributing.md

@@ -327,6 +327,10 @@ general rules of thumb can be helpful in navigating how to structure changes:
   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).
+
 - **Code should only be moved to a central location (e.g., //base) when
   multiple consumers would benefit.** We should resist the temptation to
   build overly generic common libraries as that can lead to code bloat and