GitLab

Adds an CanExceedIdleDeadlineIfRequired function to RendererScheduler
to enable IdleTasks to estimate the best time for them to exceed their deadline
if they have a large monolithic task which could not be run otherwise.

Long idle task design doc:
https://docs.google.com/a/chromium.org/document/d/1yBlUdYW8VTIfB-DqhvQqUeP0kf-Ap1W4cao2yQq58Do/edit

BUG=455713

Review URL: https://codereview.chromium.org/969373002

Cr-Commit-Position: refs/heads/master@{#320817}