Add NULL GL context implementation. Use in bench (-nullgl) and SampleApp (backspace key)

Review URL: http://codereview.appspot.com/5303080/


git-svn-id: http://skia.googlecode.com/svn/trunk@2545 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/gpu/SkMesaGLContext.h b/include/gpu/SkMesaGLContext.h
index b7578f3..5c329ff 100644
--- a/include/gpu/SkMesaGLContext.h
+++ b/include/gpu/SkMesaGLContext.h
@@ -38,7 +38,7 @@
 
 protected:
     virtual const GrGLInterface* createGLContext() SK_OVERRIDE;
-    void destroyGLContext() SK_OVERRIDE;
+    virtual void destroyGLContext() SK_OVERRIDE;
 
 private:
     Context fContext;