Vulkan: Correctly request 16bit shader features.

All the querying of extensions was correctly handled for the WebGPU
ShaderFloat16 extensions, except that we forgot to request it when
creating the VkDevice.

In practice this doesn't crash as most drivers won't check if the
extension is enabled, and just compile the shader code correctly.
However the Vulkan Validation Layers complain. This CL fixes it by
requesting the features on device creation.

Bug: dawn:426
Change-Id: Id7307bb4f1d08a10bec9fc7bf0500c5761f96ff7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/23203
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

src/dawn_native/vulkan/DeviceVk.cpp

@@ -324,6 +324,11 @@ namespace dawn_native { namespace vulkan {
 
             usedKnobs.shaderFloat16Int8Features.shaderFloat16 = VK_TRUE;
             usedKnobs._16BitStorageFeatures.uniformAndStorageBuffer16BitAccess = VK_TRUE;
+
+            featuresChain.Add(&usedKnobs.shaderFloat16Int8Features,
+                              VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SHADER_FLOAT16_INT8_FEATURES_KHR);
+            featuresChain.Add(&usedKnobs._16BitStorageFeatures,
+                              VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_16BIT_STORAGE_FEATURES);
         }
 
         // Find a universal queue family