Default initialize all descriptors

Some dawn_unittests crash on some configurations because the
uninitialized |label| member crashed string serialization.
Default initialize all descriptors to avoid this problem.

Bug: none
Change-Id: I6ea1851ebb6f54690a28ba396e0beaa85d8670cc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16260
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/fuzzers/DawnWireServerFuzzer.cpp

@@ -52,9 +52,7 @@ namespace {
     WGPUSwapChain ErrorDeviceCreateSwapChain(WGPUDevice device,
                                              WGPUSurface surface,
                                              const WGPUSwapChainDescriptor*) {
-        WGPUSwapChainDescriptor desc;
-        desc.nextInChain = nullptr;
-        desc.label = nullptr;
+        WGPUSwapChainDescriptor desc = {};
         // A 0 implementation will trigger a swapchain creation error.
         desc.implementation = 0;
         return sOriginalDeviceCreateSwapChain(device, surface, &desc);