Disable forwarding tasks to the Blink scheduler

Disable forwarding tasks to the Blink scheduler to avoid some
regressions which it has introduced.

BUG=391005,415758,415478,412714,416362,416827,417608
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/609483002

Cr-Commit-Position: refs/heads/master@{#296916}