Auto merge of #6183 - ecoal95:webglcontextattributes, r=nox

r? @jdm

I couldn't add the `getContextAttributes` method since `CodegenRust`
doesn't know how to return a dictionary value, I'll take a look at it ASAP.

I think the helper functions can return directly the renderer, since they're used just for that, but I wanted to hear your opinions about this.

By the way I'm interested in adding more serious tests for WebGL, and I think the [khronos conformance suit](https://github.com/KhronosGroup/WebGL/tree/master/conformance-suites/1.0.3) should be the best option.

Should I try to integrate it in wpt, or making a `tests/webgl` directory (or similar) inside the servo tree? (Maybe this question should be for @Ms2ger)

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components/script/dom/webidls/HTMLCanvasElement.webidl

@@ -12,7 +12,7 @@ interface HTMLCanvasElement : HTMLElement {
   [Pure]
            attribute unsigned long height;
 
-  RenderingContext? getContext(DOMString contextId);
+  RenderingContext? getContext(DOMString contextId, any... arguments);
   //boolean probablySupportsContext(DOMString contextId, any... arguments);
 
   //void setContext(RenderingContext context);

components/script/dom/webidls/WebGLRenderingContext.webidl

@@ -481,7 +481,7 @@ interface WebGLRenderingContextBase
     //readonly attribute GLsizei drawingBufferWidth;
     //readonly attribute GLsizei drawingBufferHeight;
 
-    //[WebGLHandlesContextLoss] WebGLContextAttributes? getContextAttributes();
+    [WebGLHandlesContextLoss] WebGLContextAttributes? getContextAttributes();
     //[WebGLHandlesContextLoss] boolean isContextLost();
 
     //sequence<DOMString>? getSupportedExtensions();