Let the process have more than one thread under ThreadSanitizer in RenderSandboxHostLinux::Init

content/browser/renderer_host/render_sandbox_host_linux.cc

Index: content/browser/renderer_host/render_sandbox_host_linux.cc
===================================================================
--- content/browser/renderer_host/render_sandbox_host_linux.cc	(revision 247659)
+++ content/browser/renderer_host/render_sandbox_host_linux.cc	(working copy)
@@ -709,10 +709,11 @@
   childs_lifeline_fd_ = pipefds[1];
 
   // We need to be monothreaded before we fork().
-#if !defined(TOOLKIT_GTK)
+#if !defined(TOOLKIT_GTK) && !defined(THREAD_SANITIZER)
   // Exclude gtk port as TestSuite in base/tests/test_suite.cc is calling
   // gtk_init.
-  // TODO(oshima): Remove ifdef when above issues are resolved.
+  // TODO(oshima): Remove the TOOLKIT_GTK ifdef when above issues are resolved.
+  // ThreadSanitizer spawns its own background thread.
   DCHECK_EQ(1, base::GetNumberOfThreads(base::GetCurrentProcessHandle()));
 #endif
   pid_ = fork();