[content/test] Customise the default trace output file.

When --enable-tracing-output=FILE is not provided, use a filename which
has some information about the test this trace was coming from and the
result of the test.

This allows us to easily trace multiple tests at the same time and trace
repeated runs of the same test when debugging flakes.

R=sky@chromium.org,eseckler@chromium.org
BUG=976795

Change-Id: I578a38f4c5be30e8e38cc0ddf8847e0e30ffc64c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1667667
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675636}

content/public/test/browser_test_base.cc

@@ -21,6 +21,7 @@
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_loop_current.h"
+#include "base/rand_util.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
@@ -526,6 +527,31 @@ void BrowserTestBase::WaitUntilJavaIsReady(base::OnceClosure quit_closure) {
 }
 #endif
 
+namespace {
+
+std::string GetDefaultTraceFilaneme() {
+  std::string test_suite_name = ::testing::UnitTest::GetInstance()
+                                    ->current_test_info()
+                                    ->test_suite_name();
+  std::string test_name =
+      ::testing::UnitTest::GetInstance()->current_test_info()->name();
+  // Add random number to the trace file to distinguish traces from different
+  // test runs.
+  // We don't use timestamp here to avoid collisions with parallel runs of the
+  // same test.
+  std::string random_seed = base::NumberToString(base::RandInt(1e7, 1e8 - 1));
+  std::string status = ::testing::UnitTest::GetInstance()
+                               ->current_test_info()
+                               ->result()
+                               ->Passed()
+                           ? "OK"
+                           : "FAIL";
+  return "trace_test_" + test_suite_name + "_" + test_name + "_" + random_seed +
+         "_" + status + ".json";
+}
+
+}  // namespace
+
 void BrowserTestBase::ProxyRunTestOnMainThreadLoop() {
   // Install a RunLoop timeout if none is present but do not override tests that
   // set a ScopedRunTimeoutForTest from their fixture's constructor (which
@@ -611,7 +637,7 @@ void BrowserTestBase::ProxyRunTestOnMainThreadLoop() {
     // If there was no file specified, put a hardcoded one in the current
     // working directory.
     if (trace_file.empty())
-      trace_file = base::FilePath().AppendASCII("trace.json");
+      trace_file = base::FilePath().AppendASCII(GetDefaultTraceFilaneme());
 
     // Wait for tracing to collect results from the renderers.
     base::RunLoop run_loop;