dawn_native: Factor getting procs into a single function.

Now the backends only expose the creation of devices. Getting the procs
is moved to DawnNative that will over time more backend-agnostic APIs.

src/include/dawn_native/VulkanBackend.h

@@ -24,9 +24,8 @@
 #include <vector>
 
 namespace dawn_native { namespace vulkan {
-    DAWN_NATIVE_EXPORT void Init(dawnProcTable* procs,
-                                 dawnDevice* device,
-                                 const std::vector<const char*>& requiredInstanceExtensions);
+    DAWN_NATIVE_EXPORT dawnDevice
+    CreateDevice(const std::vector<const char*>& requiredInstanceExtensions);
 
     DAWN_NATIVE_EXPORT VkInstance GetInstance(dawnDevice device);