headless: Fix handling of replies without any parameters

Fix handling of replies to DevTools commands which don't include any
parameters. Previously we used th presence of the reply dictionary to
decide between a callback with or without parameters, but this
dictionary can be present in the reply also when there are no actual
parameters in it.

BUG=595353

Review URL: https://codereview.chromium.org/1898633006

Cr-Commit-Position: refs/heads/master@{#388543}

headless/lib/browser/headless_devtools_client_impl.cc

@@ -95,10 +95,14 @@ void HeadlessDevToolsClientImpl::DispatchProtocolMessage(
     NOTREACHED() << "Unexpected reply";
     return;
   }
-  base::DictionaryValue* result_dict;
-  if (message_dict->GetDictionary("result", &result_dict)) {
+  if (!it->second.callback_with_result.is_null()) {
+    base::DictionaryValue* result_dict;
+    if (!message_dict->GetDictionary("result", &result_dict)) {
+      NOTREACHED() << "Badly formed reply result";
+      return;
+    }
     it->second.callback_with_result.Run(*result_dict);
-  } else {
+  } else if (!it->second.callback.is_null()) {
     it->second.callback.Run();
   }
   pending_messages_.erase(it);

headless/lib/headless_devtools_client_browsertest.cc

@@ -110,4 +110,39 @@ class HeadlessDevToolsClientEvalTest : public HeadlessDevToolsClientTest {
 
 DEVTOOLS_CLIENT_TEST_F(HeadlessDevToolsClientEvalTest);
 
+class HeadlessDevToolsClientCallbackTest : public HeadlessDevToolsClientTest {
+ public:
+  HeadlessDevToolsClientCallbackTest() : first_result_received_(false) {}
+
+  void RunDevToolsClientTest() override {
+    // Null callback without parameters.
+    devtools_client_->GetRuntime()->Run();
+    // Null callback with parameters.
+    devtools_client_->GetRuntime()->Evaluate("true");
+    // Non-null callback without parameters.
+    devtools_client_->GetRuntime()->Disable(
+        base::Bind(&HeadlessDevToolsClientCallbackTest::OnFirstResult,
+                   base::Unretained(this)));
+    // Non-null callback with parameters.
+    devtools_client_->GetRuntime()->Evaluate(
+        "true", base::Bind(&HeadlessDevToolsClientCallbackTest::OnSecondResult,
+                           base::Unretained(this)));
+  }
+
+  void OnFirstResult() {
+    EXPECT_FALSE(first_result_received_);
+    first_result_received_ = true;
+  }
+
+  void OnSecondResult(std::unique_ptr<runtime::EvaluateResult> result) {
+    EXPECT_TRUE(first_result_received_);
+    FinishAsynchronousTest();
+  }
+
+ private:
+  bool first_result_received_;
+};
+
+DEVTOOLS_CLIENT_TEST_F(HeadlessDevToolsClientCallbackTest);
+
 }  // namespace headless