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/tests/GLInterfaceValidation.cpp b/tests/GLInterfaceValidation.cpp
index 758f303..1308120 100755
--- a/tests/GLInterfaceValidation.cpp
+++ b/tests/GLInterfaceValidation.cpp
@@ -20,6 +20,7 @@
 #if SK_MESA
         {GrGLCreateMesaInterface, "Mesa"},
 #endif
+        {GrGLCreateNullInterface, "Null"},
     };
 
     // On some platforms GrGLCreateNativeInterface will fail unless an OpenGL