Add a CDP test that a newly opened window is paused early enough

... for a user-agent override to be applied.

BUG=1070568

Change-Id: Ifba5b72b15d9edd2f14e9a0c7f9860e15fa6f3c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628313
Auto-Submit: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845002}

third_party/blink/web_tests/http/tests/inspector-protocol/target/target-setAutoAttach-windowOpen-ua-override-expected.txt

+Tests that browser.Target.setAutoAttach() attaches to window.open targets.
+Opened the window
+Attached to window, waitingForDebugger=true
+got: User-Agent: Lynx v0.1
+

third_party/blink/web_tests/http/tests/inspector-protocol/target/target-setAutoAttach-windowOpen-ua-override.js

+(async function(testRunner) {
+  const {page, session, dp} = await testRunner.startBlank(
+      `Tests that browser.Target.setAutoAttach() attaches to window.open targets.`);
+
+  const target = testRunner.browserP().Target;
+  await target.setDiscoverTargets({discover: true});
+  await target.setAutoAttach({autoAttach: true, waitForDebuggerOnStart: true, flatten: true});
+
+  session.evaluate(`
+    window.myWindow = window.open('/inspector-protocol/emulation/resources/echo-headers.php');
+    undefined;
+  `);
+  testRunner.log('Opened the window');
+  const attachedEvent = await target.onceAttachedToTarget();
+  testRunner.log('Attached to window, waitingForDebugger=' + attachedEvent.params.waitingForDebugger);
+  const popupSession = new TestRunner.Session(testRunner, attachedEvent.params.sessionId);
+  const changedPromise = target.onceTargetInfoChanged();
+  popupSession.protocol.Page.enable();
+  await popupSession.protocol.Emulation.setUserAgentOverride({userAgent: 'Lynx v0.1'});
+  popupSession.protocol.Runtime.runIfWaitingForDebugger();
+  await popupSession.protocol.Page.onceLoadEventFired();
+  const textContent = await popupSession.evaluate(`document.body.textContent`);
+  const userAgent = textContent.split('\n').find(line => /^User-Agent:/.test(line));
+  testRunner.log(`got: ${userAgent}`);
+  testRunner.completeTest();
+})