Clarify artifact documentation for test results

This clarifies the meaning of forward slashes in artifact file paths,
and also clarifies the directory these paths are relative to, when no
artifact permanent location is set.

Bug: 775619
Change-Id: I1f41d86273455123823350a39b239cc04b6a0216
Reviewed-on: https://chromium-review.googlesource.com/802092
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521525}

docs/testing/json_test_results_format.md

@@ -107,7 +107,7 @@ results for each invocation in the `actual` field.
 |-------------|-----------|-------------|
 |  `actual` | string | **Required.** An ordered space-separated list of the results the test actually produced. `FAIL PASS` means that a test was run twice, failed the first time, and then passed when it was retried. If a test produces multiple different results, then it was actually flaky during the run. |
 |  `expected` | string | **Required.** An unordered space-separated list of the result types expected for the test, e.g. `FAIL PASS` means that a test is expected to either pass or fail. A test that contains multiple values is expected to be flaky. |
-|  `artifacts` | dict | **Optional.** A dictionary describing test artifacts generated by the execution of the test. The dictionary maps the name of the artifact (`screenshot`, `crash_log`) to a list of relative locations of the artifact (`screenshot/page.png`, `logs/crash.txt`). There is one entry in the list per test execution. If `artifact_permanent_location` is specified, then this location is relative to that path. Otherwise, it is assumed that this test file is in a known location by whatever is processing this test file, and so the location is relative to a known directory. |
+|  `artifacts` | dict | **Optional.** A dictionary describing test artifacts generated by the execution of the test. The dictionary maps the name of the artifact (`screenshot`, `crash_log`) to a list of relative locations of the artifact (`screenshot/page.png`, `logs/crash.txt`). Any '/' characters in the file paths are meant to be platform agnostic; tools will replace them with the appropriate per platform path separators. There is one entry in the list per test execution. If `artifact_permanent_location` is specified, then this location is relative to that path. Otherwise, the path is assumed to be relative to the location of the json file which contains this. |
 |  `bugs` | string | **Optional.** A comma-separated list of URLs to bug database entries associated with each test. |
 |  `is_unexpected` | bool | **Optional.** If present and true, the failure was unexpected (a regression). If false (or if the key is not present at all), the failure was expected and will be ignored. |
 |  `time` | float | **Optional.** If present, the time it took in seconds to execute the first invocation of the test. |