scheduler: Remove WebSchedulerProxy

This class is no longer needed in anticipation of moving the scheduler
from Blink to the content layer.

BUG=391005

Review URL: https://codereview.chromium.org/664703002

git-svn-id: svn://svn.chromium.org/blink/trunk@184105 bbb929c8-8fbe-4397-9dbb-9b2b20218538