Add how_to_get_stack_trace_for_builders.md

This is helpful for anyone wants to find stack trace
on skylab builders.

Bug: 1380885
Change-Id: I79daf2c40721a88bd0ca7b10bc46a3baf5e0ce54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4811704
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1188035}

docs/lacros/how_to_get_stack_trace_for_builders.md

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+# How to get stack trace for Chrome crashes on DUT builders
+
+This doc is helpful for getting stack trace for skylab builders. It should
+be the same process for both Ash and Lacros. Also it should be the same process
+for Tast tests, Crosier and gtests.
+
+## From the builder page
+
+It should look like this
+![shard]
+Then you click the shard link to get to the chromeos builder page.
+
+## Get the binary
+
+On this page, on the right side 'Input properties', you should see the
+'lacrosGcsPath'.
+![gcs_path]
+That path contains all the Chrome files that needed to run the test.
+
+Note: For ash builders, the path name is still 'lacrosGcsPath'.
+
+Download the files locally:
+```
+$ gsutil cp -r <lacros_gcs_path> ~/
+```
+
+## Get the dmp file
+Click the testhaus link
+![testhaus]
+you should see the testhaus page which contains all the logs.
+![dmp_file]
+You need to find out the right dmp file. Usually if you're only debugging
+1 test failure, you will see 2 dmp files due to test retries. You can randomly
+select one to download.
+
+## Get stack trace
+This is the example commandline:
+```
+$ sudo mount lacros_compressed.squash /mnt
+$ lldb /mnt/out/Release/chromeos_integration_tests -c ~/Downloads/<some_name>.dmp
+(lldb) bt
+```
+
+For Tast test, replace the target from chromeos_integration_tests to chrome.
+
+Note: Make sure you use the matching binary and dmp file. e.g. You don't want
+to use the Ash chrome binary with a Lacros dmp file.
+
+Final note: This doc is only for getting stack trace for the component under
+test. That means if your builder build Lacros and lacros crashes, or build Ash
+and Ash crashes, this workflow is helpful. But if your builder build Lacros and
+causes Ash crashes, the workflow need to change. In theory you can get the
+binary from the OS image under test, and dmp file in the same way and then get
+stack trace. The author of this doc have not tried this so please help improve
+this doc if you have information on this.
+
+[shard]: images/shard.png
+[testhaus]: images/testhaus.png
+[dmp_file]: images/dmp_file.png
+[gcs_path]: images/gcs_path.png