Throttle JavaScript timers in prerender tabs.

For energy usage, invisible tabs should have a lower timer threshold.
Based on file history, it looks like prerender was not considered when this logic was introduced.

BUG=400343

Review URL: https://codereview.chromium.org/431503005

git-svn-id: svn://svn.chromium.org/blink/trunk@179618 bbb929c8-8fbe-4397-9dbb-9b2b20218538