Add a GetInstanceProcAddr helper for the native Vulkan backend.

This is a helper to allow a Dawn embedder to look up the Vulkan device
functions.  Possibly remove when we design a cleaner way to share function
pointers.

Change-Id: I3ed92568e543c1aad9e0e64d72ad0990824640cb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12980
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_native/vulkan/VulkanBackend.cpp

@@ -33,6 +33,12 @@ namespace dawn_native { namespace vulkan {
         return backendDevice->GetVkInstance();
     }
 
+    DAWN_NATIVE_EXPORT PFN_vkVoidFunction GetInstanceProcAddr(WGPUDevice device,
+                                                              const char* pName) {
+        Device* backendDevice = reinterpret_cast<Device*>(device);
+        return (*backendDevice->fn.GetInstanceProcAddr)(backendDevice->GetVkInstance(), pName);
+    }
+
     // Explicitly export this function because it uses the "native" type for surfaces while the
     // header as seen in this file uses the wrapped type.
     DAWN_NATIVE_EXPORT DawnSwapChainImplementation

src/include/dawn_native/VulkanBackend.h

@@ -34,6 +34,8 @@ namespace dawn_native { namespace vulkan {
 
     DAWN_NATIVE_EXPORT VkInstance GetInstance(WGPUDevice device);
 
+    DAWN_NATIVE_EXPORT PFN_vkVoidFunction GetInstanceProcAddr(WGPUDevice device, const char* pName);
+
     DAWN_NATIVE_EXPORT DawnSwapChainImplementation CreateNativeSwapChainImpl(WGPUDevice device,
                                                                              VkSurfaceKHR surface);
     DAWN_NATIVE_EXPORT WGPUTextureFormat