Allow --enable-tracing-output to take a directory name.

PROBLEM: It is not possible to retrieve traces from swarming with
--gtest_repeat.

For a file to be retrievable from swarming, it must be placed in
${ISOLATED_OUTDIR} but specifying anything for --enable-tracing-output
disables the unique-filename-per-test behaviour.

With this change it becomes possible to do e.g.

tools/mb/mb.py run ... \
  --gtest_filter=WebNavigationApiTest.Crash \
  --gtest_repeat=100
  --enable-tracing=*,disabled-by-default-toplevel.flow \
  --enable-tracing-output='${ISOLATED_OUTDIR}/'

and have unique files for each test that are retrievable with swarming
collect.

Also drive-by fix typo in name of function.

BUG=1043616

Change-Id: Idd1bcbbdb6f18d297170f074e9409c61204fcacb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445649
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814052}

content/public/common/content_switches.cc

@@ -408,7 +408,9 @@ const char kEnableThreadedCompositing[]     = "enable-threaded-compositing";
 // Enable tracing during the execution of browser tests.
 const char kEnableTracing[]                 = "enable-tracing";
 
-// The filename to write the output of the test tracing to.
+// The filename to write the output of the test tracing to. If it is empty
+// or it ends in a directory separator then an auto-generated filename will be
+// appended.
 const char kEnableTracingOutput[]           = "enable-tracing-output";
 
 // Enable screen capturing support for MediaStream API.

content/public/test/browser_test_base.cc

@@ -654,7 +654,7 @@ void BrowserTestBase::WaitUntilJavaIsReady(base::OnceClosure quit_closure) {
 
 namespace {
 
-std::string GetDefaultTraceFileneme() {
+std::string GetDefaultTraceFilename() {
   std::string test_suite_name = ::testing::UnitTest::GetInstance()
                                     ->current_test_info()
                                     ->test_suite_name();
@@ -769,10 +769,10 @@ void BrowserTestBase::ProxyRunTestOnMainThreadLoop() {
     base::FilePath trace_file =
         base::CommandLine::ForCurrentProcess()->GetSwitchValuePath(
             switches::kEnableTracingOutput);
-    // If there was no file specified, put a hardcoded one in the current
-    // working directory.
-    if (trace_file.empty())
-      trace_file = base::FilePath().AppendASCII(GetDefaultTraceFileneme());
+    // If |trace_file| ends in a directory separator or is empty use a generated
+    // name in that directory (empty means current directory).
+    if (trace_file.empty() || trace_file.EndsWithSeparator())
+      trace_file = trace_file.AppendASCII(GetDefaultTraceFilename());
 
     // Wait for tracing to collect results from the renderers.
     base::RunLoop run_loop;