Add Instance and CreateInstance to webgpu.h

This is the first step in making the API before WGPUDevice creation
match webgpu.h and is necessary to implement WGPUSwapChain.

BUG=dawn:269

Change-Id: If92ced42d7683d79e67c02738949ff8b483d22c4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14061
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/Instance.cpp

@@ -50,6 +50,19 @@ namespace dawn_native {
 
     // InstanceBase
 
+    // static
+    InstanceBase* InstanceBase::Create(const InstanceDescriptor* descriptor) {
+        Ref<InstanceBase> instance = AcquireRef(new InstanceBase);
+        if (!instance->Initialize(descriptor)) {
+            return nullptr;
+        }
+        return instance.Detach();
+    }
+
+    bool InstanceBase::Initialize(const InstanceDescriptor*) {
+        return true;
+    }
+
     void InstanceBase::DiscoverDefaultAdapters() {
         EnsureBackendConnections();