Revert workaround for Opera bug related to base element and pushState.

The bug about BASE element being erroneously eliminated by
history.pushState() is long fixed.
Optimize state saving on scrolling.

Update history state 1.5s after scrolling has stopped and not on each
scroll event.

Fix is targeting Chrome primarily due to its favicon flickering
madness on calling pushState(). Especially on Mac, where scrolling is
per-pixel, the favicon flickering becomes very noticeable.