Allow throwing an exception in fulfilled function in WaitUntilObserver

Previously, when an exception happens in an internal callback to fulfill a
promise passed to waitUntil or respondWith, it invokes rejected callback too,
though we expect that either of those is called. This CL correct it by using
ScriptPromise::Then() twice.
Previously, we used ScriptPromise::Then() as like Then(f, g), and it means
calling a promise.then(f).catch(g); in the JavaScript world. I changed it to
Then(f, _), Then(_, g), which is equivalent to promise.then(f);
promise.catch(g);.

Bug: 889567
Change-Id: Id2e2848762b50df633293571c01c5f60d106de7c
Reviewed-on: https://chromium-review.googlesource.com/c/1312141Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604535}

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

@@ -182,10 +182,16 @@ void WaitUntilObserver::WaitUntil(ScriptState* script_state,
   }
 
   IncrementPendingPromiseCount();
+
+  // Call Then() separately for fulfilled and rejected cases. This ensures
+  // throwing an exception in |on_promise_fulfilled| doesn't invoke
+  // |on_promise_rejected|.
   script_promise.Then(
       ThenFunction::CreateFunction(script_state, this, ThenFunction::kFulfilled,
                                    std::move(on_promise_fulfilled)),
-      ThenFunction::CreateFunction(script_state, this, ThenFunction::kRejected,
+      {});
+  script_promise.Then({}, ThenFunction::CreateFunction(
+                              script_state, this, ThenFunction::kRejected,
                               std::move(on_promise_rejected)));
 }