Adds the concept of Policies to the Blink Scheduler

In Normal mode (the default), there's virtually no 
prioritization of compositor events, and
shouldYieldForHighPriorityWork always returns false. In 
CompositorPriority mode, there *is* prioritization of 
compositor tasks and shouldYieldForHighPriorityWork will 
return false if there are pending high priority events.

Currently there is no way for client code to set the policy
but that will be introduced in a follow up CL.

Also adds some disabled by default tracing in
"blink.scheduler" to track the number of pending high
priority tasks and which policy mode the scheduler is in.

BUG=391005,411520

Review URL: https://codereview.chromium.org/559973003

git-svn-id: svn://svn.chromium.org/blink/trunk@181818 bbb929c8-8fbe-4397-9dbb-9b2b20218538