width_offset and height_offset should never be negative (#46724)

Reftests all failing on Mac because windows size is set to 0. The reason
windows size is zero is because width_offset and height_offset are
negative -800 and -600 respectively, and when they are added to the
expected size, the final size is 0.

This seems to be some mac side syncronazation issue. Add this sanite
check to bypass this issue for now.

Bug: https://g-issues.chromium.org/issues/339909604

Author: WeizhongX

tools/wptrunner/wptrunner/executors/executorwebdriver.py

@@ -696,6 +696,9 @@ def do_test(self, test):
             """return [window.outerWidth - window.innerWidth,
                        window.outerHeight - window.innerHeight];"""
         )
+        # width_offset and height_offset should never be negative
+        width_offset = max(width_offset, 0)
+        height_offset = max(height_offset, 0)
         try:
             self.protocol.webdriver.window.position = (0, 0)
         except error.InvalidArgumentException: