Retire ResizeObserver runtime flag

ResizeObserver shipped to stable in M64
https://chromium-review.googlesource.com/754562
https://www.chromestatus.com/features/5705346022637568

BUG=612962

Change-Id: I8b323722c3bf8caa028ec5d14964b43dc13d1faa
Reviewed-on: https://chromium-review.googlesource.com/1163060
Reviewed-by: Aleks Totic <atotic@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581054}
3 files changed