Rename Pause/Unpaused to ContextPaused/ContextUnpaused.

Add a enumeration state of the type of pausing of tasks the
ExecutionContext has.

kPaused is the state where only tasks are paused.
kFrozen is the state where all things on the page should be paused
including media.

BUG=907125

Change-Id: Id72209f25bdc7d512d6cc41b02636c2bdadb54a3
Reviewed-on: https://chromium-review.googlesource.com/c/1409812
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623256}
29 files changed