interactive-ui-tests: Assert if trace is not started

Add an assert to catch the case where the derived tests
forgot to call PerformanceTest::SetUpOnMainThread to start
tracing.

Bug: 945709
Change-Id: Id3a659f172f5fde09220003b7a3fc57f18c832fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566610
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650516}

chrome/test/base/perf/performance_test.cc

@@ -57,6 +57,9 @@ void PerformanceTest::SetUpOnMainThread() {
 void PerformanceTest::TearDownOnMainThread() {
   if (should_start_trace_) {
     auto* controller = content::TracingController::GetInstance();
+    ASSERT_TRUE(controller->IsTracing())
+        << "Did you forget to call PerformanceTest::SetUpOnMainThread?";
+
     base::RunLoop runloop;
     base::FilePath dir =
         base::CommandLine::ForCurrentProcess()->GetSwitchValuePath(kTraceDir);