Enable D3D12 validation layers

Run end2end tests with argument '-d' or '--enable-backend-validation'

Change-Id: I34a3f453dcd3a57d76301801ae2abe42f847cb1b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7140
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Hao Li <hao.x.li@intel.com>

src/dawn_native/DawnNative.cpp

@@ -99,4 +99,11 @@ namespace dawn_native {
         return mImpl->GetToggleInfo(toggleName);
     }
 
+    void Instance::EnableBackendValidation(bool enableBackendValidation) {
+        mImpl->EnableBackendValidation(enableBackendValidation);
+    }
+
+    bool Instance::IsBackendValidationEnabled() {
+        return mImpl->IsBackendValidationEnabled();
+    }
 }  // namespace dawn_native