[tracing] (try to) deflake StartupTracingTest x2.

Rewrite WaitForCondition to call RunLoop::Run only once and avoid
relying on an explicit timeout.

Combined with crrev.com/c/2616018 this should eliminate timeouts on
asan/tsan bots, where the tests are known to take more than the old
timeout of 30 seconds.

R=eseckler@chromium.org
BUG=1157954,1041392

Change-Id: Idb880b64a69b9c5323a683a2764549452345924a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615351
Commit-Queue: Alexander Timin <altimin@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Auto-Submit: Alexander Timin <altimin@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841547}

content/browser/tracing/startup_tracing_browsertest.cc

@@ -28,17 +28,27 @@ namespace content {
 
 namespace {
 
+void CheckForConditionAndWaitMoreIfNeeded(
+    base::RepeatingCallback<bool()> condition,
+    base::OnceClosure quit_closure) {
+  if (condition.Run()) {
+    std::move(quit_closure).Run();
+    return;
+  }
+  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
+      FROM_HERE,
+      base::BindOnce(&CheckForConditionAndWaitMoreIfNeeded,
+                     std::move(condition), std::move(quit_closure)),
+      TestTimeouts::tiny_timeout());
+}
+
 // Wait until |condition| returns true.
 void WaitForCondition(base::RepeatingCallback<bool()> condition,
                       const std::string& description) {
-  const base::TimeDelta kTimeout = base::TimeDelta::FromSeconds(30);
-  const base::TimeTicks start_time = base::TimeTicks::Now();
-  while (!condition.Run() && (base::TimeTicks::Now() - start_time < kTimeout)) {
   base::RunLoop run_loop;
-    base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
-        FROM_HERE, run_loop.QuitClosure(), TestTimeouts::tiny_timeout());
+  CheckForConditionAndWaitMoreIfNeeded(condition, run_loop.QuitClosure());
   run_loop.Run();
-  }
+
   ASSERT_TRUE(condition.Run())
       << "Timeout waiting for condition: " << description;
 }