cc: Remove unnecessary task runner refptrs from schedulers.

Task runners are ref counted so that they can be used safely from
multiple threads. This is not required for (Display)Scheduler in which
case the owner of the scheduler (Proxy or OnScreenDisplayClient) holds
a reference to the task runner for as long as the scheduler lives.

R=brianderson,mithro
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1205483004

Cr-Commit-Position: refs/heads/master@{#335817}

• cc/scheduler/scheduler.cc[diff]
• cc/scheduler/scheduler.h[diff]
• cc/scheduler/scheduler_unittest.cc[diff]
• cc/surfaces/display_scheduler.cc[diff]
• cc/surfaces/display_scheduler.h[diff]
• cc/surfaces/display_scheduler_unittest.cc[diff]
• cc/surfaces/display_unittest.cc[diff]
• cc/surfaces/onscreen_display_client.cc[diff]
• cc/test/scheduler_test_common.cc[diff]
• cc/test/scheduler_test_common.h[diff]
• components/view_manager/surfaces/surfaces_scheduler.cc[diff]