updating_clang.md: Add Pinpoint perf tryjob instruction

Bug: none
Change-Id: I3622122b52039f04028627505ca432b154cad4da
Reviewed-on: https://chromium-review.googlesource.com/1104345
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567998}

docs/updating_clang.md

@@ -43,6 +43,35 @@
     git cl try -m luci.chromium.try -b linux-chromeos-dbg
     ```
 
+1.  Start Pinpoint perf tryjobs. These are generally too noisy to catch minor
+    regressions pre-commit, but make sure there are no large regressions.
+
+    a.  (Log in to store OAuth2 token in the depot_tools cache. Only needs to be
+        run once:)
+
+        ```shell
+        $ PYTHONPATH=$(dirname $(which git-cl)) python -c"import auth;auth.OAUTH_CLIENT_ID='62121018386-h08uiaftreu4dr3c4alh3l7mogskvb7i.apps.googleusercontent.com';auth.OAUTH_CLIENT_SECRET='vc1fZfV1cZC6mgDSHV-KSPOz';print auth.get_authenticator_for_host('pinpoint',auth.make_auth_config()).login()"
+        ```
+
+    a.  Generate a fresh Oath2 token:
+
+        ```shell
+        $ TOKEN=$(PYTHONPATH=$(dirname $(which git-cl)) python -c"import auth;print auth.get_authenticator_for_host('pinpoint',auth.make_auth_config()).get_access_token().token")
+        ```
+
+    a.  Launch Pinpoint job:
+
+        ```shell
+        $ curl -H"Authorization: Bearer $TOKEN" -F configuration=chromium-rel-win7-gpu-nvidia \
+            -F target=telemetry_perf_tests -F benchmark=speedometer2 \
+            -F patch=https://chromium-review.googlesource.com/c/chromium/src/+/$(git cl issue | cut -d' ' -f3) \
+            -F start_git_hash=HEAD -F end_git_hash=HEAD https://pinpoint-dot-chromeperf.appspot.com/api/new
+        ```
+
+    a.  Use the URL returned by the command above to see the progress and result
+        of the tryjob, checking that it doesn't regress significantly (> 10%).
+        Post the URL to the codereview.
+
 1.  Commit roll CL from the first step
 1.  The bots will now pull the prebuilt binary, and goma will have a matching
     binary, too.