Enable Root Document Level Passive Event Listeners

Switch flag to enabled by default. Approved intent to implement:

https://groups.google.com/a/chromium.org/d/msg/blink-dev/BW3qrkisqIs/v5Au-HVTAwAJ

BUG=625675

Review-Url: https://codereview.chromium.org/2501073002
Cr-Commit-Position: refs/heads/master@{#432130}
3 files changed