Add gpu implementation of OverdrawXfermode

src/gpu/glsl/GrGLSLXferProcessor.h

Index: src/gpu/glsl/GrGLSLXferProcessor.h
diff --git a/src/gpu/glsl/GrGLSLXferProcessor.h b/src/gpu/glsl/GrGLSLXferProcessor.h
index 6886740584de7ae768e976dfada613912ddcbafe..37e684fcde320b84a0019b62f4e4d74e4c3e0ea6 100644
--- a/src/gpu/glsl/GrGLSLXferProcessor.h
+++ b/src/gpu/glsl/GrGLSLXferProcessor.h
@@ -75,7 +75,7 @@ protected:
                                           const char* outColor,
                                           const char* outColorSecondary,
                                           const GrXferProcessor& proc);
-    
+
 private:
     /**
      * Called by emitCode() when the XP will not be performing a dst read. This method is