Update maps_pixel_test docs

The maps_pixel_test makes use of Skia Gold Triage links, but not only as an
artifact. Not actual baselines.

For those unfamiliar this can be confusing, as the lack of positive image in
Skia, is juxtaposed with there only being a json of expectations in code search.

To clarify this I'm adding some docs to the maps_integration_test.py to have a
note that the actual pixel failures will be reported in the logs, and that the
Skia Gold Triage is a supplimentary artifact.

TEST=maps_pixel_test

Change-Id: I1c802ba2d4d021890333cbe9c857e54824a0476a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095207
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748391}

content/test/gpu/gpu_tests/maps_integration_test.py

@@ -59,6 +59,14 @@ class MapsIntegrationTest(
   @classmethod
   def GenerateGpuTests(cls, options):
     cls.SetParsedCommandLineOptions(options)
+    # The maps_pixel_expectations.json contain the actual image expectations. If
+    # the test fails, with errors greater than the tolerance for the run, then
+    # the logs will report the actual failure.
+    #
+    # There will also be a Skia Gold Triage link, this will be used to store the
+    # artifact of the failure to help with debugging. There are no accepted
+    # positive baselines recorded in Skia Gold, so its diff will not be
+    # sufficient to debugging the failure.
     yield('Maps_maps',
           'file://performance.html',
           ('maps_pixel_expectations.json'))