Expose the full API for attaching devtools clients:

headless/lib/headless_devtools_client_browsertest.cc

Index: headless/lib/headless_devtools_client_browsertest.cc
diff --git a/headless/lib/headless_devtools_client_browsertest.cc b/headless/lib/headless_devtools_client_browsertest.cc
index cce4c045a9e5cbe33ba0a82be1f0231ce212a7ac..521c8b19253bb0565f3fe4cce1a2224a60d08e83 100644
--- a/headless/lib/headless_devtools_client_browsertest.cc
+++ b/headless/lib/headless_devtools_client_browsertest.cc
@@ -687,4 +687,63 @@ class HeadlessCrashObserverTest : public HeadlessAsyncDevTooledBrowserTest,
 
 HEADLESS_ASYNC_DEVTOOLED_TEST_F(HeadlessCrashObserverTest);
 
+class HeadlessDevToolsClientAttachTest
+    : public HeadlessAsyncDevTooledBrowserTest {
+ public:
+  void RunDevTooledTest() override {
+    other_devtools_client_ = HeadlessDevToolsClient::Create();
+    HeadlessDevToolsTarget* devtools_target =
+        web_contents_->GetDevToolsTarget();
+
+    // Try attaching: there's already a client attached.
+    EXPECT_FALSE(devtools_target->AttachClient(other_devtools_client_.get()));
+    EXPECT_TRUE(devtools_target->IsAttached());
+    // Detach the existing client, attach the other client.
+    devtools_target->DetachClient(devtools_client_.get());
+    EXPECT_FALSE(devtools_target->IsAttached());
+    EXPECT_TRUE(devtools_target->AttachClient(other_devtools_client_.get()));
+    EXPECT_TRUE(devtools_target->IsAttached());
+
+    // Now, let's make sure this devtools client works.
+    other_devtools_client_->GetRuntime()->Evaluate(
+        "24 * 7", base::Bind(&HeadlessDevToolsClientAttachTest::OnFirstResult,
+                             base::Unretained(this)));
+  }
+
+  void OnFirstResult(std::unique_ptr<runtime::EvaluateResult> result) {
+    int value;
+    EXPECT_TRUE(result->GetResult()->HasValue());
+    EXPECT_TRUE(result->GetResult()->GetValue()->GetAsInteger(&value));
+    EXPECT_EQ(24 * 7, value);
+
+    HeadlessDevToolsTarget* devtools_target =
+        web_contents_->GetDevToolsTarget();
+
+    // Try attach, then force-attach the original client.
+    EXPECT_FALSE(devtools_target->AttachClient(devtools_client_.get()));
+    devtools_target->ForceAttachClient(devtools_client_.get());
+    EXPECT_TRUE(devtools_target->IsAttached());
+
+    devtools_client_->GetRuntime()->Evaluate(
+        "27 * 4", base::Bind(&HeadlessDevToolsClientAttachTest::OnSecondResult,
+                             base::Unretained(this)));
+  }
+
+  void OnSecondResult(std::unique_ptr<runtime::EvaluateResult> result) {
+    int value;
+    EXPECT_TRUE(result->GetResult()->HasValue());
+    EXPECT_TRUE(result->GetResult()->GetValue()->GetAsInteger(&value));
+    EXPECT_EQ(27 * 4, value);
+
+    // If everything worked, this call will not crash, since it
+    // detaches devtools_client_.
+    FinishAsynchronousTest();
+  }
+
+ protected:
+  std::unique_ptr<HeadlessDevToolsClient> other_devtools_client_;
+};
+
+HEADLESS_ASYNC_DEVTOOLED_TEST_F(HeadlessDevToolsClientAttachTest);
+
 }  // namespace headless