docs: Add note about dawn_perf_tests without a Chromium checkout

Building from a Chromium checkout with all the optimization flags
is most useful when LTO makes a signficant performance difference.

This patch updates the docs to be clear that testing performance can
still be done without a Chromium checkout. This is the easiest path
for external partners.

Bug: dawn:23
Change-Id: I6ee0f0a94f14f12973d0ef4c62298278f11fb7a0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14503
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
diff --git a/docs/testing.md b/docs/testing.md
index 3b40d37..20d715f 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -11,6 +11,8 @@
 use_cfi_icall=false       # Required because Dawn dynamically loads function pointers, and we don't sanitize them yet.
 ```
 
+A Chromium checkout is required for the highest optimization flags. It is possible to build and run `dawn_perf_tests` from a standalone Dawn checkout as well, only using GN arg `is_debug=false`. For more information on building, please see [building.md](./building.md).
+
 ### Terminology
 
  - Iteration: The unit of work being measured. It could be a frame, a draw call, a data upload, a computation, etc. `dawn_perf_tests` metrics are reported as time per iteration.