[Blink-WebWorkers] WorkerSharedTimer cancels extra delayed tasks.

This CL has the Blink side changes to cancel extra delayed tasks posted
in WorkerSharedTimer. Previously extra tasks were being posted and allowed
to complete if multiple interval timers were scheduled on worker threads.
Now we will cancel any extra delayed task, made redundant due to newly scheduled
lesser timer interval tasks.

BUG=410786

Review URL: https://codereview.chromium.org/563203002

git-svn-id: svn://svn.chromium.org/blink/trunk@182930 bbb929c8-8fbe-4397-9dbb-9b2b20218538