Fix exit code emitted by wptrunner.

Stop setting flag --no-fail-on-unexpected, which causes wptrunner to
always exit with code 0. Omitting this flag makes the exit code equal
the number of failed tests. The CQ needs the exit code in order to
determine which shards to retry.

Bug: 937369
Change-Id: I150321a7783948724be0d622646108e5df6249eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992911Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730163}

testing/scripts/run_wpt_tests.py

@@ -55,7 +55,11 @@ class WPTTestAdapter(common.BaseIsolatedScriptArgsAdapter):
             "--no-capture-stdio",
             "--no-manifest-download",
             "--no-pause-after-test",
-            "--no-fail-on-unexpected",
+            # Setting --no-fail-on-unexpected makes the return code of wpt 0
+            # even if there were test failures. The CQ doesn't like this since
+            # it uses the exit code to determine which shards to retry (ie:
+            # those that had non-zero exit codes).
+            #"--no-fail-on-unexpected",
             "--metadata",
             WPT_METADATA_DIR,
             # By specifying metadata above, WPT will try to find manifest in the