Remove the post-processing step from run_wpt_tests.py

This is carved off from crrev.com/c/2072639 to get around false positive
on linux_layout_tests_composite_after_paint

Bug: 937369
Change-Id: I0a968b13d36c4037ca7aeed20793afb000967aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080466Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Auto-Submit: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746014}

testing/buildbot/gn_isolate_map.pyl

@@ -527,11 +527,6 @@
     "label": "//:wpt_tests_isolate",
     "type": "script",
     "script": "//testing/scripts/run_wpt_tests.py",
-    "args": [
-      "--old-json-output-file-path=${ISOLATED_OUTDIR}/output.json",
-      "--new-json-output-dir=${ISOLATED_OUTDIR}",
-      "--new-json-output-filename=output.json",
-    ],
   },
   "chrome/installer/linux": {
     "label": "//chrome/installer/linux:linux",

testing/scripts/run_wpt_tests.py

@@ -86,13 +86,6 @@ class WPTTestAdapter(common.BaseIsolatedScriptArgsAdapter):
         ])
         return rest_args
 
-    def add_extra_arguments(self, parser):
-        # These args are used to rewrite the output generated by WPT to include
-        # missing features, such as flakineess expectations.
-        parser.add_argument("--old-json-output-file-path")
-        parser.add_argument("--new-json-output-dir")
-        parser.add_argument("--new-json-output-filename")
-
 
 def main():
     # First, generate WPT metadata files.