Re-enable ProtoBackgroundTracingTest.ProtoTraceReceived

The non-allowlisted proto fields that caused this to flake should
be allowed again now.

R=ssid@chromium.org
BUG=1008387

Change-Id: I829bc29e5e5d7fc9f4856c57f651f84c4bdfd1a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436672
Commit-Queue: ssid <ssid@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812410}

content/browser/tracing/background_tracing_manager_browsertest.cc

@@ -1775,9 +1775,7 @@ IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest,
   background_tracing_helper.WaitForScenarioAborted();
 }
 
-// TODO(1008387): Disabled for flakiness.
-IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest,
-                       DISABLED_ProtoTraceReceived) {
+IN_PROC_BROWSER_TEST_F(ProtoBackgroundTracingTest, ProtoTraceReceived) {
   TestBackgroundTracingHelper background_tracing_helper;
 
   std::unique_ptr<BackgroundTracingConfig> config = CreatePreemptiveConfig();