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/include/dawn_native/DawnNative.h

@@ -146,6 +146,9 @@ namespace dawn_native {
 
         void SetPlatform(dawn_platform::Platform* platform);
 
+        // Returns the underlying WGPUInstance object.
+        WGPUInstance Get() const;
+
       private:
         InstanceBase* mImpl = nullptr;
     };