DevTools: introduce WebInspector.Throttler

This patch introduces WebInspector.Throttler and uses it for throttling
calls to PageAgent in OverridesSupport, as well as throttling calls in StylesSourceMapping.

Throttler is a helper object that throttles execution of processes
(possibly asynchronous).
Throttler is created with a single parameter - throttle interval T.
Throttler satisfies to the following contract: for every two consecutive
runs performed by throttler, time between the end of the first run and
start of the second run is always greater or equal to T.

Throttler.schedule has additional second parameter "asSoonAsPossible" which essentially
sets throttler timeout into 0 for the next process run.

Review URL: https://codereview.chromium.org/319143002

git-svn-id: svn://svn.chromium.org/blink/trunk@175956 bbb929c8-8fbe-4397-9dbb-9b2b20218538