[Documentation] Initial documentation on flaky Web Tests

This lays out a skeleton for the document, plus fills in some details of
how to go from a build results page to the test that flaked.

Bug: 1127369
Change-Id: Icea0ef102d694a61cf8f390c563ce108afee0553
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405418
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815262}

docs/testing/web_tests_addressing_flake.md

+# Addressing Flaky Web Tests
+
+## Understanding builder results
+
+Often (e.g. by Flake Portal), you will be pointed to a particular build in which
+your test has flaked. You will need the name of the specific build step that has
+flaked; usually for Web Tests this is `blink_web_tests` but there are variations
+(e.g. `not_site_per_process_blink_web_tests`).
+
+On the builder page, find the appropriate step:
+
+![web_tests_blink_web_tests_step]
+
+While you can examine the individual shard logs to find your test output, it is
+easier to view the consolidated information, so scroll down to the **archive
+results for blink\_web\_tests** step and click the `layout_test_results` link:
+
+![web_tests_archive_blink_web_tests_step]
+
+This will open a new tab with the results viewer. By default your test should be
+shown, but if it isn't then you can click the 'All' button in the 'Query' row,
+then enter the test filename in the textbox beside 'Filters':
+
+![web_tests_results_viewer_query_filter]
+
+There are a few ways that a Web Test can flake, and what the result means may
+depend on the [test type](writing_web_tests.md#Test-Types):
+
+1. `FAIL` - the test failed. For reference or pixel tests, this means it did not
+   match the reference image. For JavaScript tests, the test either failed an
+   assertion *or* did not match the [baseline](web_test_expectations.md)
+   `-expected.txt` file checked in for it.
+   * For image tests, this status is reported as `IMAGE` (as in an image diff).
+   * For Javascript tests, this status is reported as `TEXT` (as in a text
+     diff).
+1. `TIMEOUT` - the test timed out before producing a result. This may happen if
+   the test is slow and normally runs close to the timeout limit, but is usually
+   caused by waiting on an event that never happens. These unfortunately [do not
+   produce any logs](https://crbug.com/487051).
+1. `CRASH` - the browser crashed while executing the test. There should be logs
+   associated with the crash available.
+1. `PASS` - this can happen! Web Tests can be marked as [expected to
+   fail](web_test_expectations.md), and if they then pass then that is an
+   unexpected result, aka a potential flake.
+
+Clicking on the test row anywhere *except* the test name (which is a link to the
+test itself) will expand the entry to show information about the failure result,
+including actual/expected results and browser logs if they exist.
+
+In the following example, our flaky test has a `FAIL` result which is a flake
+compared to its (default) expected `PASS` result. The test results (`TEXT` - as
+explained above this is equivalent to `FAIL`), output, and browser log links are
+highlighted.
+
+![web_tests_results_viewer_flaky_test]
+
+## Reproducing Web Test flakes
+
+>TODO: add documentation on reproducing Web Test flakes
+
+## Debugging flaky Web Tests
+
+>TODO: add documentation on debugging flaky Web Tests
+
+[web_tests_blink_web_tests_step]: images/web_tests_blink_web_tests_step.png
+[web_tests_archive_blink_web_tests_step]: images/web_tests_archive_blink_web_tests_step.png
+[web_tests_results_viewer_query_filter]: images/web_tests_results_viewer_query_filter.png
+[web_tests_results_viewer_flaky_test]: images/web_tests_results_viewer_flaky_test.png