[run_wpt_tests.py] Move WPT manifest files gen to after argument parsing

This is mostly a code cleanup; it seemed weird that if you pass '--help'
to the script it would generate the manifest files before showing you
the command line!

Bug: None
Change-Id: I00515216c6883ee38d591993a59bf9d960163b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412695Reviewed-by: Luke Z <lpz@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807558}

testing/scripts/common.py

@@ -324,6 +324,9 @@ class BaseIsolatedScriptArgsAdapter(object):
   def clean_up_after_test_run(self):
     pass
 
+  def do_pre_test_run_tasks(self):
+    pass
+
   def do_post_test_run_tasks(self):
     pass
 
@@ -331,6 +334,8 @@ class BaseIsolatedScriptArgsAdapter(object):
     self.parse_args()
     cmd = self.generate_isolated_script_cmd()
 
+    self.do_pre_test_run_tasks()
+
     env = os.environ.copy()
 
     # Assume we want to set up the sandbox environment variables all the

testing/scripts/run_wpt_tests.py

@@ -80,19 +80,21 @@ class WPTTestAdapter(wpt_common.BaseWptScriptAdapter):
         ])
         return rest_args
 
-def main():
-    # First, generate WPT metadata files.
-    common.run_command([
-        sys.executable,
-        os.path.join(wpt_common.BLINK_TOOLS_DIR, 'build_wpt_metadata.py'),
-        "--metadata-output-dir",
-        WPT_METADATA_OUTPUT_DIR,
-        "--additional-expectations",
-        WPT_OVERRIDE_EXPECTATIONS_PATH,
-        "--checked-in-metadata-dir",
-        WPT_CHECKED_IN_METADATA_DIR
-    ])
+    def do_pre_test_run_tasks(self):
+        # Generate WPT metadata files.
+        common.run_command([
+            sys.executable,
+            os.path.join(wpt_common.BLINK_TOOLS_DIR, 'build_wpt_metadata.py'),
+            "--metadata-output-dir",
+            WPT_METADATA_OUTPUT_DIR,
+            "--additional-expectations",
+            WPT_OVERRIDE_EXPECTATIONS_PATH,
+            "--checked-in-metadata-dir",
+            WPT_CHECKED_IN_METADATA_DIR
+        ])
 
+
+def main():
     adapter = WPTTestAdapter()
     return adapter.run_test()