scheduler: Make DOM window event tasks unthrottled

This patch avoids throttling events that are routed via
DOMWindowEventQueue. This affects a handful of window-related events,
the most important ones being IndexedDB events. It turns out many web
pages (e.g., Google Docs) rely on IndexedDB transactions to be
relatively fast and start encountering errors if they are throttled too
aggressively while a tab is in the background.

BUG=675372

Review-Url: https://codereview.chromium.org/2651933002
Cr-Commit-Position: refs/heads/master@{#446000}
6 files changed