Update pixel wrangling doc

Add instructions for rubber stamping expectations.

Bug: 1159542
Change-Id: Ifc546f0e8915ed2291322d4af38a8066c40ac728
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611667
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840309}

docs/gpu/pixel_wrangling.md

@@ -271,6 +271,10 @@ shift, and a calendar appointment.
         how important it is to eliminate flakiness rather than hiding it.
     1. For failures of rendering_representative_perf_tests please refer to its
     [instructions on updating expectations][rendering_representative_perf_tests].
+    1. It's always better to have the CL reviewed properly, but for urgent
+       suppressions when no reviewer is available, it is possible to rubber
+       stamp the CL via adding `rubber-stamper@appspot.gserviceaccount.com` as
+       your reviewer, in addition to the regular reviewer.
 1.  For the remaining Gtest-style tests, use the [`DISABLED_`
     modifier][gtest-DISABLED] to suppress any failures if necessary.