Add bb extensions for scroll* attributes that don't adjust for zooming

The zoom adjustment results in rounding errors that cause visual stuttering
while scrolling.  These methods return the raw scroll values without any zoom
adjustments.  We will adjust for zoom manually at the JS level.
Make bbScrollLeftNoZoomAdjust and bbScrollTopNoZoomAdjust settable

This is so that we could avoid rounding on set scrollLeft/scrollTop.
Add bbWindowRectChanged event

This event is fired on the window object when Blink receives an updated window
screen rect.  This event can be used by content to update itself based on the
new screen coordinates.