We need to add getContextAttributes(). Possibly more
This bug is now just for adding getContextAttributes.
I'm closing this as a duplicate of https://bugs.webkit.org/show_bug.cgi?id=31169 because if getContextAttributes() isn't added at the same time as support for getContext("webgl", ...attributes...), no tests of WebGLContextAttributes can be written.
*** This bug has been marked as a duplicate of bug 31169 ***