tracing: Bail out early in tracing tests when tracing is already enabled

content/browser/tracing/memory_tracing_browsertest.cc

Index: content/browser/tracing/memory_tracing_browsertest.cc
diff --git a/content/browser/tracing/memory_tracing_browsertest.cc b/content/browser/tracing/memory_tracing_browsertest.cc
index 80ccfae3df249b4667b6a5de6a5c51bc9194b0ca..ab9b9e3f67d3307d91f5866f63f11bbe439900bd 100644
--- a/content/browser/tracing/memory_tracing_browsertest.cc
+++ b/content/browser/tracing/memory_tracing_browsertest.cc
@@ -13,6 +13,7 @@
 #include "base/trace_event/memory_dump_provider.h"
 #include "base/trace_event/memory_dump_request_args.h"
 #include "base/trace_event/trace_config_memory_test_util.h"
+#include "base/trace_event/trace_log.h"
 #include "content/public/browser/tracing_controller.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/test/browser_test_utils.h"
@@ -109,6 +110,9 @@ class MemoryTracingTest : public ContentBrowserTest {
   }
 
   void EnableMemoryTracing() {
+    CHECK(!base::trace_event::TraceLog::GetInstance()->IsEnabled())
+        << "Tracing seems to be already enabled. Very likely this is because "
+           "the startup tracing file has been leaked from a previous test.";
     // Enable tracing without periodic dumps.
     base::trace_event::TraceConfig trace_config(
         base::trace_event::TraceConfigMemoryTestUtil::