Adjust heightForString div offscreen.

The div used in SC.heightForString is left in the DOM. If the most recent
height measured was taller than the window, then in cases where the body
element's overflow was set to auto, the scrollable height would match
this div's height.

To avoid this, after we determine the height, set the div's top to -height.
Fix issue with clippingFrame not being properly updated.
Ensure menu pane position updates correctly.

Menu pane position was not always updating correctly when maximizing
the browser.
Clean up SC.Benchmark.
Bumping to SC 1.9.0.15.