DevTools: [Throttler] run the first task instantly

The first task the throttler will run will be run after 0 timeout.
This will avoid having perceived latency when throttler is used for
defered UI rendering.

R=pfeldman

Review URL: https://codereview.chromium.org/681083003

git-svn-id: svn://svn.chromium.org/blink/trunk@184710 bbb929c8-8fbe-4397-9dbb-9b2b20218538