Expose GetBaseline() method on RenderText.

ui/gfx/render_text.h

Index: ui/gfx/render_text.h
===================================================================
--- ui/gfx/render_text.h	(revision 143588)
+++ ui/gfx/render_text.h	(working copy)
@@ -246,12 +246,16 @@
   // |GetTextDirection()|, not the direction of a particular run.
   VisualCursorDirection GetVisualDirectionOfLogicalEnd();
 
-  // Get the size in pixels of the entire string. For the height, this will
+  // Returns the size in pixels of the entire string. For the height, this will
   // return the maximum height among the different fonts in the text runs.
   // Note that this returns the raw size of the string, which does not include
   // the margin area of text shadows.
   virtual Size GetStringSize() = 0;
 
+  // Returns the common baseline of the text. The returned value is the vertical
+  // offset from the top of |display_rect| to the text baseline, in pixels.
+  virtual int GetBaseline() = 0;
+
   void Draw(Canvas* canvas);
 
   // Gets the SelectionModel from a visual point in local coordinates.