Implement move by word, fix rendering of Arabic shape joining

ui/gfx/render_text_linux.h

Index: ui/gfx/render_text_linux.h
===================================================================
--- ui/gfx/render_text_linux.h	(revision 100777)
+++ ui/gfx/render_text_linux.h	(working copy)
@@ -74,6 +74,11 @@
   SelectionModel LeftSelectionModel(const SelectionModel& current);
   SelectionModel RightSelectionModel(const SelectionModel& current);
 
+  // Get the selection model visually left or right of |current| by one word.
+  // The returned value represents a cursor/caret position without a selection.
+  SelectionModel LeftSelectionModelByWord(const SelectionModel& current);
+  SelectionModel RightSelectionModelByWord(const SelectionModel& current);
+
   // If |layout_| is NULL, create and setup |layout_|, retain and ref
   // |current_line_|. Return |layout_|.
   PangoLayout* EnsureLayout();