Avoid file leaks in ios tests in /ios/chrome

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R=rohitrao@chromium.org

Bug: 546640
Change-Id: I0743e2c24e68d42e962cc671139a0c17ffb127c6
Fixed: 1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991753
Commit-Queue: Oliver Li <olivierli@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Auto-Submit: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730019}

ios/chrome/browser/test/perf_test_with_bvc_ios.mm

@@ -167,6 +167,10 @@ void PerfTestWithBVC::TearDown() {
     SpinRunLoop(.5);
   PerfTest::TearDown();
 
+  // Before destroying chrome_browser_state_ we need to make sure that no tasks
+  // are left on the ThreadPool since they might depend on it.
+  task_environment_.RunUntilIdle();
+
   // The profiles can be deallocated only after the BVC has been deallocated.
   chrome_browser_state_.reset();
 }