Frames; simplify logic.

Simplifications:
  - We don't need to rotate the array, because we're adding the new
    count to the index of the current frame.
  - We don't need the extra logic in unregisterFrame to handle the case
    of the tab closing.  The chrome.tabs.onRemoved listener takes care
    of that.
  - In nextFrame, we don't need to worry about frameId not being in the
    list.  How could we get here withtout the frame being registered?
  - Use a global `currentRequest` variable.