Set colorAttachment.clearColor to NAN in CHelloTriangle.cpp

This patch sets the deprecated member colorAttachment.clearColor to
NAN in CHelloTriangle.cpp to eliminate a warning when running the
demo.

Bug: dawn:1269
Change-Id: I274198d1cafbc2dc15dcf6e59d26b58a62134c5a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/106400
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn/samples/CHelloTriangle.cpp

@@ -107,6 +107,7 @@ void frame() {
         colorAttachment.view = backbufferView;
         colorAttachment.resolveTarget = nullptr;
         colorAttachment.clearValue = {0.0f, 0.0f, 0.0f, 0.0f};
+        colorAttachment.clearColor = {NAN, NAN, NAN, NAN};
         colorAttachment.loadOp = WGPULoadOp_Clear;
         colorAttachment.storeOp = WGPUStoreOp_Store;
         renderpassInfo.colorAttachmentCount = 1;