Simplify WaitUntilObserver::ThenFunction::Call

It was previously doing three ScriptValue copy assignment operator calls:
first from ScriptPromise's promise_ to a temporary object, then to
the value object, and thirdly to the function's return object.

Using return value optimization, the new code only does one copy
assignment: directly to the return object.

It's not a big deal, but that assignment operator is non-trivial,
so we might as well not call it more than necessary.

Bug: none
Change-Id: Idd5544b51a776ac5491c29e0ed5898484135f1ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512884
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822977}

third_party/blink/renderer/modules/service_worker/wait_until_observer.cc

@@ -93,12 +93,13 @@ class WaitUntilObserver::ThenFunction final : public ScriptFunction {
       event_loop->EnqueueMicrotask(
           WTF::Bind(&WaitUntilObserver::OnPromiseRejected,
                     WrapPersistent(observer_.Get())));
-      value = ScriptPromise::Reject(GetScriptState(), value).GetScriptValue();
-    } else {
-      event_loop->EnqueueMicrotask(
-          WTF::Bind(&WaitUntilObserver::OnPromiseFulfilled,
-                    WrapPersistent(observer_.Get())));
+      observer_ = nullptr;
+      return ScriptPromise::Reject(GetScriptState(), value).GetScriptValue();
     }
+
+    event_loop->EnqueueMicrotask(
+        WTF::Bind(&WaitUntilObserver::OnPromiseFulfilled,
+                  WrapPersistent(observer_.Get())));
     observer_ = nullptr;
     return value;
   }