Support ErrorScopes for asynchronous GPU execution

This changes updates ErrorScopes so that scopes enclosing a
Queue::Submit or Queue::Signal resolve their callbacks asynchronously
after GPU execution is complete.

Bug: dawn:153
Change-Id: I0e0b8a9f19f3f29d1b6a3683938154b87f190a07
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/10701
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_native/ErrorScope.cpp

@@ -111,4 +111,11 @@ namespace dawn_native {
         }
     }
 
+    void ErrorScope::Destroy() {
+        if (!IsRoot()) {
+            mErrorType = dawn::ErrorType::Unknown;
+            mErrorMessage = "Error scope destroyed";
+        }
+    }
+
 }  // namespace dawn_native