We want AccessibleCaret be of the same size regardless of the zoom
level. We simply divide the caret's width, height, margin-left, and the
text selection bar's margin-left by current zoom level.
The margin-left of the caret is adjusted from -23px to -23.5px for better
looking.
See AccessibleCaret.h for the class description.
Technical difference between AccessibleCaret and Touch/SelectionCarets:
The anonymous dom element containing a caret image will be created by
AccessibleCaret by using the API landed in bug 1020244 instead of being
created by nsCanvasFrame.
