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
b49f2bb5dd