Fix Workers continue execution ignoring all breakpoints after 3 seconds

WorkerRunLoop::runInMode should not exit if the timeout was caused by
idle watchdog.

BUG=265847

Review URL: https://chromiumcodereview.appspot.com/21090005

git-svn-id: svn://svn.chromium.org/blink/trunk@155685 bbb929c8-8fbe-4397-9dbb-9b2b20218538