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/include/dawn_native/DawnNative.h

@@ -125,6 +125,10 @@ namespace dawn_native {
 
         const ToggleInfo* GetToggleInfo(const char* toggleName);
 
+        // Enable backend's validation layers if it has.
+        void EnableBackendValidation(bool enableBackendValidation);
+        bool IsBackendValidationEnabled();
+
       private:
         InstanceBase* mImpl = nullptr;
     };