scheduler: Post high priority tasks directly to the message loop

This patch removes the scheduler's internal high priority task queues in
favor of posting high priority tasks directly to the main thread. This
fixes re-entrancy problems with running these tasks in a nested message
loop, but removes the option of having them jump ahead of other posted
tasks.

This is an interim solution while we research better ways to prioritize
compositor tasks over other (Blink and non-Blink) clients of the main
thread message loop.

BUG=391005

Review URL: https://codereview.chromium.org/621363002

git-svn-id: svn://svn.chromium.org/blink/trunk@183287 bbb929c8-8fbe-4397-9dbb-9b2b20218538