Add runtime search paths to Instance descriptor

So that Chrome can configure Dawn to load SwiftShader from
the bundled/module directory. The shared libraries and ICD
are packaged in a separate directory while dawn::native is
linked statically into the Chrome executable.

Change the Vulkan backend to use these paths for loading
Vulkan.

Bug: chromium:1266550
Change-Id: I40468b481881f6c249694c1c61137bc0c9b8fe76
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/78840
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/include/dawn_native/DawnNative.h

@@ -136,7 +136,7 @@ namespace dawn::native {
     // for this instance.
     class DAWN_NATIVE_EXPORT Instance {
       public:
-        Instance();
+        explicit Instance(const WGPUInstanceDescriptor* desc = nullptr);
         ~Instance();
 
         Instance(const Instance& other) = delete;