Allow ResultOrError to downcast from backend to frontend types

More concretely this makes Result<T*, E*> able to be move-constructed
from Result<ChildClassOfT*, E*> for free.

BUG=dawn:19

Change-Id: Iea2b8997079ac3bfcf270d6b73a79cf5cac2c06f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/11860
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/ResultTests.cpp

@@ -139,6 +139,31 @@ TEST(ResultBothPointer, ReturningSuccess) {
     TestSuccess(&result, &dummySuccess);
 }
 
+// Tests converting from a Result<TChild*, E*>
+TEST(ResultBothPointer, ConversionFromChildClass) {
+    struct T {
+        int a;
+    };
+    struct TChild : T {};
+
+    TChild child;
+    T* childAsT = &child;
+    {
+        Result<T*, int*> result(&child);
+        TestSuccess(&result, childAsT);
+    }
+    {
+        Result<TChild*, int*> resultChild(&child);
+        Result<T*, int*> result(std::move(resultChild));
+        TestSuccess(&result, childAsT);
+    }
+    {
+        Result<TChild*, int*> resultChild(&child);
+        Result<T*, int*> result = std::move(resultChild);
+        TestSuccess(&result, childAsT);
+    }
+}
+
 // Result<const T*, E*>
 
 // Test constructing an error Result<const T*, E*>