[mojo] Update DocumentInterfaceBroker web test according to guidelines

This change replaces then() with await() as suggested in a previous review.
It also moves the first assert to a point after the override is set to verify
that calls on previously created proxies are not affected by setting the
override.

Bug: 718652
Change-Id: Ia0ac1e1793503b8548cac8e8ef468f22eac9d934
Reviewed-on: https://chromium-review.googlesource.com/c/1481578Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634485}

third_party/blink/web_tests/mojo/document-interface-broker-override.html

@@ -18,14 +18,8 @@ promise_test(async t => {
       Mojo.getDocumentInterfaceBrokerHandle());
 
   const testInterfaceProxyBeforeOverride = new blink.mojom.FrameHostTestInterfaceProxy;
-  console.log("proxy" + typeof(testInterfaceProxyBeforeOverride));
   brokerProxy.getFrameHostTestInterface(testInterfaceProxyBeforeOverride.$.createRequest());
 
-  // Verify that RenderFrameHostImpl's implementation gets called without an override
-  testInterfaceProxyBeforeOverride.getName().then(reply => {
-    assert_equals(reply.name, 'RenderFrameHostImpl');
-  });
-
   setDocumentInterfaceBrokerOverrides({ getFrameHostTestInterface: request => {
     frameHostTestImpl.bindHandle(request.handle);
   }});
@@ -33,10 +27,13 @@ promise_test(async t => {
   const testInterfaceProxyAfterOverride = new blink.mojom.FrameHostTestInterfaceProxy;
   brokerProxy.getFrameHostTestInterface(testInterfaceProxyAfterOverride.$.createRequest());
 
+  // Verify that RenderFrameHostImpl's implementation gets called without an override
+  let reply = await testInterfaceProxyBeforeOverride.getName();
+  assert_equals(reply.name, 'RenderFrameHostImpl');
+
   // Verify that the test implementation gets called after the override
-  return testInterfaceProxyAfterOverride.getName().then(reply => {
-    assert_equals(reply.name, 'TestFrameHostTestImpl');
-  });
+  reply = await testInterfaceProxyAfterOverride.getName();
+  assert_equals(reply.name, 'TestFrameHostTestImpl');
 },
 'Appropriate DocumentInterfaceBroker implementations are called before and after overriding');