Change from left/right to base/extent for selection handles.

 Bug 6557816
 This is a step in preparation to fix text area selection.
 It moves selection from left/right to base/extent. left/right
 can cause problems when dragging cursors -- it is sometimes
 difficult to tell which cursor should be dragged, the left or
 right, but we can always keep the base/extent constant.

 Webkit Change: I5609d07ea2ee67fc08d15cfcad31f49302f33297

Change-Id: I2f9b963e93bb1ab86a9b26518fca52449b56d5d9