Disable BackgroundTracingManagerBrowserTest.EarlyTraceEventsInTrace.

This test is still flaky. Error:
```
Value of: trace_receiver_helper.TraceHasMatchingString("TestEarlyEvent")
  Actual: false
Stack trace:
```

Expected: true
Tbr: oysteine@chromium.org
Bug: 1534867
Change-Id: Id5c1111d1e5dc8dfc97b09c835caae24457bf28c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541179Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644767}

content/browser/tracing/background_tracing_manager_browsertest.cc

@@ -436,8 +436,9 @@ IN_PROC_BROWSER_TEST_F(BackgroundTracingManagerBrowserTest,
 // the full WaitForTracingEnabled() callback (background tracing will directly
 // enable the TraceLog so we get events prior to waiting for the whole IPC
 // sequence to enable tracing coming back from the tracing service).
+// TODO(https://crbug.com/945484): This tracing test is flaky.
 IN_PROC_BROWSER_TEST_F(BackgroundTracingManagerBrowserTest,
-                       EarlyTraceEventsInTrace) {
+                       DISABLED_EarlyTraceEventsInTrace) {
   TestTraceReceiverHelper trace_receiver_helper;
   TestBackgroundTracingHelper background_tracing_helper;