Introduce PushTask() to unify StartEvent() and PushPendingTask()

This is a pure refactoring, as a preparation for introducing a concept
of 'offline' tasks later.

Background: The WIP CL, https://crrev.com/c/1900803/, introduced a
concept of a *Task* in ServiceWorkerTimeoutTimer, however, there are
still two different queues in the timer, |pending_tasks_| (for normal
tasks) and |pending_after_idle_timeout_tasks_| (for pending
after-idle-timeout tasks). That are confusing and error-prone.

So before proceeding further, I've decided to refactor the current
ServiceWorkerTimeoutTimer, merging two different interfaces,
StartEvent() and PushPendingTask(), into one interface, PushTask().

Now all callers should use PushTask(). The current StartEvent(),
StartEventWithCustomCustomTimeout(), and PushPendingTask() are no
longer available from outside. StartEvent() is now an internal
function, and others were just removed.

After this CL, I'll introduce a new kind of a task, OFFLINE.

Bug: 965802
Change-Id: Ide270432f486484c1ac2127cff59e0258f88a835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906452
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720386}
5 files changed