Change selection to use left/right instead of base/extent.

 Bug 5859620
 Simplify text selection to use left/right rather than
 base/extent. In combination with webkit change, this
 fixes RTL selection. The handle directions are properly-
 facing when the text direction is the same for both
 sides of selection.
 Webkit Change: I3362c5034ce08ce1517a86882c13b78aff35fe3f

Change-Id: Ib88ed2327182ba5b47b3e41584cbe944d05c8ada