Update GPU pixel wrangling documentation

Updates the GPU pixel wrangling documentation to point out
//content/test/gpu/trim_culprit_cls.py as a potentially useful tool and
updates the script's documentation to include example usages.

Bug: 1120104
Change-Id: I2026e994408097bfee8b94984dc89e8110afba0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373391
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801232}

content/test/gpu/trim_culprit_cls.py

@@ -8,6 +8,29 @@ Script for determining which CLs in a blamelist ran on a certain trybot.
 There are cases where CLs can be absolved of a CI failure if they ran on a
 similar trybot before being submitted. This CL will go through each CL in a
 given blamelist and determine whether they ran on a specified trybot or not.
+
+This script depends on the `bq` tool, which is available as part of the Google
+Cloud SDK https://cloud.google.com/sdk/docs/quickstarts.
+
+Example usage:
+
+trim_culprit_cls.py \
+  --start-revision <first/oldest revision in the blamelist> \
+  --end-revision <last/newest revision in the blamelist> \
+  --trybot <optional trybot name> \
+  --project <billing project>
+
+Concrete example:
+
+trim_culprit_cls.py \
+  --start-revision 1cdf916d194215f1e4139f295e494fc1c1863c3c \
+  --end-revision 9aa31419100be8d0f02708a500aaed7c33a53a10 \
+  --trybot win_optional_gpu_tests_rel \
+  --project chromium-swarm
+
+The --project argument can be any project you are associated with in the
+Google Cloud console https://console.cloud.google.com/ (see drop-down menu in
+the top left corner).
 """
 
 import argparse

docs/gpu/pixel_wrangling.md

@@ -167,6 +167,12 @@ shift, and a calendar appointment.
         revert message, provide a clear description of what broke, links to
         failing builds, and excerpts of the failure logs, because the build
         logs expire after a few days.
+    1.  If the failure is one that you believe should have been caught by an
+        optional GPU trybot, you can use the script at
+        [`//content/test/gpu/trim_culprit_cls.py`][trim culprit cls] to help
+        trim down the blamelist by finding out which CLs passed said trybot
+        before submission. See the documentation at the top of the script for
+        example usage, etc.
 1.  Make sure the bots are running jobs.
     1.  Keep an eye on the console views of the various bots.
     1.  Make sure the bots are all actively processing jobs. If they go offline
@@ -269,6 +275,7 @@ shift, and a calendar appointment.
     modifier][gtest-DISABLED] to suppress any failures if necessary.
 
 [Sheriff-O-Matic]: https://sheriff-o-matic.appspot.com/chromium.gpu
+[trim culprit cls]: https://source.chromium.org/chromium/chromium/src/+/master:content/test/gpu/trim_culprit_cls.py
 [tree sheriffing page]: https://sites.google.com/a/chromium.org/dev/developers/tree-sheriffs
 [linux-rel]: https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-rel
 [luci.chromium.try]: https://ci.chromium.org/p/chromium/g/luci.chromium.try/builders