Turn on usage of spvc by default

The CTS has been changed to use the version of SPIR-V that spvc is
expecting, so the issues that previously caused a rollback should no
longer occur.

BUG=dawn:337

Change-Id: Ied5c39059d417359a697930287728faf57289c62
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16900
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/Device.cpp

@@ -696,6 +696,7 @@ namespace dawn_native {
     void DeviceBase::SetDefaultToggles() {
         // Sets the default-enabled toggles
         mTogglesSet.SetToggle(Toggle::LazyClearResourceOnFirstUse, true);
+        mTogglesSet.SetToggle(Toggle::UseSpvc, true);
     }
 
     // Implementation details of object creation