Fix RenderPassDescriptor to be extensible.

BUG=dawn:22

Change-Id: Ib9c2cd3259db0e07fa02134ce6d0d5d75a24546c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11901
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

dawn.json

@@ -951,6 +951,7 @@
 
     "render pass descriptor": {
         "category": "structure",
+        "extensible": true,
         "members": [
             {"name": "label", "type": "char", "annotation": "const*", "length": "strlen", "optional": true},
             {"name": "color attachment count", "type": "uint32_t"},

examples/CHelloTriangle.cpp

@@ -129,6 +129,7 @@ void frame() {
     DawnTexture backbuffer = dawnSwapChainGetNextTexture(swapchain);
     DawnTextureView backbufferView = dawnTextureCreateView(backbuffer, nullptr);
     DawnRenderPassDescriptor renderpassInfo;
+    renderpassInfo.nextInChain = nullptr;
     renderpassInfo.label = nullptr;
     DawnRenderPassColorAttachmentDescriptor colorAttachment;
     {