Revert of Eagerly dispose of ScheduledActions. (patchset #2 id:20001 of https://codereview.chromium.org/2552673002/ )

Reason for revert:
Speculative revert for reported perf decrease on system_health.memory_mobile, https://crbug.com/672098

Original issue's description:
> Eagerly dispose of ScheduledActions.
>
> The DOMTimer's ScheduledAction hold on to the script source and
> state needed to execute the timer action. Let go of ShceduledAction's
> resource early.
>
> Apart from reducing the lifetime of script source, this is a speculative
> fix for crashes reported in v8::PersistentValueVector::Clear() during
> lazy sweeping of ScheduledAction objects.
>
> R=
> BUG=
>
> Committed: https://crrev.com/11bd50343795ed1dc1977da91e9a1588687522fd
> Cr-Commit-Position: refs/heads/master@{#436298}

TBR=oilpan-reviews@chromium.org,haraken@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.chromium.org/2568103002
Cr-Commit-Position: refs/heads/master@{#437889}