Gather tracing output before tearing down the entire thread

Otherwise, the traces won't include events from renderers

R=skyostil@chromium.org

Change-Id: I40e33b3191c3e1d58a77aadf57583600286430df
Reviewed-on: https://chromium-review.googlesource.com/833880Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525327}

content/public/test/browser_test_base.cc

@@ -346,7 +346,6 @@ void BrowserTestBase::ProxyRunTestOnMainThreadLoop() {
     if (!disable_io_checks_)
       base::ThreadRestrictions::SetIOAllowed(old_io_allowed_value);
     TearDownOnMainThread();
-    PostRunTestOnMainThread();
   }
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
@@ -367,6 +366,8 @@ void BrowserTestBase::ProxyRunTestOnMainThreadLoop() {
                                    run_loop.QuitClosure(), trace_file)));
     run_loop.Run();
   }
+
+  PostRunTestOnMainThread();
 }
 
 void BrowserTestBase::CreateTestServer(const base::FilePath& test_server_base) {