Don't race on debugger paused event in breakpoint worker web test

This CL removes a race in a breakpoint web test. The CL does a similar
things an existing test helper, but in a safe way.

{waitUntilPausedAndDumpStackAndResume} also prints the reason why we
are paused. Since we pause at a breakpoint and we get a status message
for that (plus a stack trace), we removed the reason from the test
expectation in favor of a non-flaking test.

R=petermarshall@chromium.org

Change-Id: I20aa786d370e64eb16ed3a9036b5599a6be02651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028888Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736783}

third_party/blink/web_tests/http/tests/devtools/sources/debugger-breakpoints/breakpoints-ui-in-multiple-workers-expected.txt

@@ -23,16 +23,13 @@ Test breakpoint in each worker
 Script execution paused.
 Call stack:
     0) onmessage (worker.js:2)
-Paused on breakpoint
 Script execution resumed.
 Script execution paused.
 Call stack:
     0) onmessage (worker.js:2)
-Paused on breakpoint
 Script execution resumed.
 Script execution paused.
 Call stack:
     0) onmessage (worker.js:2)
-Paused on breakpoint
 Script execution resumed.
 

third_party/blink/web_tests/http/tests/devtools/sources/debugger-breakpoints/breakpoints-ui-in-multiple-workers.js

@@ -36,8 +36,13 @@
   TestRunner.addResult('Test breakpoint in each worker');
   await SourcesTestRunner.startDebuggerTestPromise();
   for (var i = 0; i < 3; ++i) {
-    TestRunner.evaluateInPageAsync(`window.workers[${i}].postMessage('')`);
-    await new Promise(resolve => SourcesTestRunner.waitUntilPausedAndDumpStackAndResume(resolve));
+    const pausedPromise = SourcesTestRunner.waitUntilPausedPromise();
+
+    await TestRunner.evaluateInPageAsync(`window.workers[${i}].postMessage('')`);
+
+    const callFrames = await pausedPromise;
+    SourcesTestRunner.captureStackTrace(callFrames);
+    await new Promise(resolve => SourcesTestRunner.resumeExecution(resolve));
   }
   SourcesTestRunner.completeDebuggerTest();