[DevTools] Add a test for sessions to headless_browsertests.

This test was originally part of
https://chromium-review.googlesource.com/c/chromium/src/+/1783717
and added there because the earlier version of the change
introduced a regression; but the test seems worthwhile
regardless of where we do the session id surgery and I figure
we may as well submit the test now.

Change-Id: If7935aa4b4d46f898233d8cee9cadca0b7835f06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1845669Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703904}

headless/test/data/protocol/sessions/headless-session-basics-expected.txt

+Tests headless session basics.
+
+SUCCESS: Target.createTarget returned sessionId.

headless/test/data/protocol/sessions/headless-session-basics.js

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(async function(testRunner) {
+  testRunner.log('Tests headless session basics.\n');
+
+  // HeadlessDevToolsSession handles Target.createTarget. So, it's responsible
+  // for adding the sessionId into its return value. This test uses the
+  // low-level browser protocol connection, attached to a browser target,
+  // and creates a browser context. In the subsequent Target.createTarget
+  // call, we observe that the return value carries the expected sessionId.
+
+  testRunner.browserP().Target.attachToBrowserTarget();
+  const sessionId =
+        (await testRunner.browserP().Target.onceAttachedToTarget())
+        .params.sessionId;
+
+  const session = new TestRunner.Session(testRunner, sessionId);
+
+  const browserContextId =
+        (await session.protocol.Target.createBrowserContext()).browserContextId;
+
+  const returnValue = (await session.protocol.Target.createTarget({
+    'browserContextId': browserContextId,
+    'enableBeginFrameControl': true, 'height': 1, 'url': 'about:blank',
+    'width': 1}));
+
+  // If HeadlessDevToolsSession fails to put the sessionId into the response,
+  // this log line will be missing.
+  if (returnValue.sessionId && returnValue.sessionId.length > 0) {
+    testRunner.log('SUCCESS: Target.createTarget returned sessionId.\n');
+  }
+
+  testRunner.completeTest();
+})

headless/test/headless_protocol_browsertest.cc

@@ -264,6 +264,9 @@ HEADLESS_PROTOCOL_TEST(MAYBE_VirtualTimeTimerSuspend,
 #undef MAYBE_VirtualTimeTimerOrder
 #undef MAYBE_VirtualTimeTimerSuspend
 
+HEADLESS_PROTOCOL_TEST(HeadlessSessionBasicsTest,
+                       "sessions/headless-session-basics.js")
+
 class HeadlessProtocolCompositorBrowserTest
     : public HeadlessProtocolBrowserTest {
  public: