[Extensions] Initial refactoring to combine task queue interfaces.

This CL unifies LazyContextTaskQueue::PendingTask and LazyBackgroundTaskQueue:PendingTask.

Change-Id: I4dac0f4494a575783781b9961345ddd48920d822
Reviewed-on: https://chromium-review.googlesource.com/c/1345531
Reviewed-by: Istiaque Ahmed <[email protected]>
Reviewed-by: Ben Wells <[email protected]>
Reviewed-by: Stuart Langley <[email protected]>
Commit-Queue: David Bertoni <[email protected]>
Cr-Commit-Position: refs/heads/master@{#617141}
16 files changed