Disable chrome overlay scrollbar animations

Overlay scrollbar animations are time delayed and not web
visible or testable. Disable timed animation delays to ensure
ref tests do not fail based on how long it takes to capture the
screenshot.

Author: Robert Flack

tools/wptrunner/wptrunner/browsers/chrome.py

@@ -158,6 +158,9 @@ def executor_kwargs(logger, test_type, test_environment, run_info_data, subsuite
         chrome_options["args"].append(
             "--ip-address-space-overrides=" + address_space_overrides_arg)
 
+    # Disable overlay scrollbar animations to prevent flaky wpt screenshots based on timing.
+    chrome_options["args"].append("--disable-features=ScrollbarAnimations")
+
     # Always disable antialiasing on the Ahem font.
     blink_features = ['DisableAhemAntialias']