fix know issues in RenderText

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 100008)
+++ ui/gfx/render_text.h	(working copy)
@@ -193,7 +193,9 @@
   // Set the selection_model_ to the value of |selection|.
   // The selection model components are modified if invalid.
   // Returns true if the cursor position or selection range changed.
-  // TODO(xji): need to check the cursor is set at grapheme boundary.
+  // If |selectin_start_| or |selection_end_| or |caret_pos_| in
+  // |selection_model| is not a cursorable position (not on grapheme boundary),
+  // it is a NO-OP and returns false.
   bool MoveCursorTo(const SelectionModel& selection_model);
 
   // Move the cursor to the position associated with the clicked point.
@@ -254,6 +256,9 @@
   // Subsequent text, cursor, or bounds changes may invalidate returned values.
   const Rect& GetUpdatedCursorBounds();
 
+  // Get the logical index of the grapheme following the argument |position|.
+  virtual size_t GetIndexOfNextGrapheme(size_t position);
+
  protected:
   RenderText();
 
@@ -288,6 +293,10 @@
   // TODO(msw) Re-evaluate this function's necessity and signature.
   virtual std::vector<Rect> GetSubstringBounds(size_t from, size_t to);
 
+  // Return true if cursor can appear in front of the character at |position|,
+  // which means it is a grapheme boundary or the first character in the text.
+  virtual bool IsCursorablePosition(size_t position) = 0;
+
   // Apply composition style (underline) to composition range and selection
   // style (foreground) to selection range.
   void ApplyCompositionAndSelectionStyles(StyleRanges* style_ranges) const;
@@ -305,16 +314,20 @@
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, ApplyStyleRange);
   FRIEND_TEST_ALL_PREFIXES(RenderTextTest, StyleRangesAdjust);
 
+  // Return an index belonging to the |next| or previous logical grapheme.
+  // The return value is bounded by 0 and the text length, inclusive.
+  virtual size_t IndexOfAdjacentGrapheme(size_t index, bool next) = 0;
+
   // Sets the selection model, the argument is assumed to be valid.
   void SetSelectionModel(const SelectionModel& selection_model);
 
   // Set the cursor to |position|, with the caret trailing the previous
   // grapheme, or if there is no previous grapheme, leading the cursor position.
   // If |select| is false, the selection start is moved to the same position.
+  // If the |position| is not a cursorable position (not on grapheme boundary),
+  // it is a NO-OP.
   void MoveCursorTo(size_t position, bool select);
 
-  bool IsPositionAtWordSelectionBoundary(size_t pos);
-
   // Update the cached bounds and display offset to ensure that the current
   // cursor is within the visible display area.
   void UpdateCachedBoundsAndOffset();