Do not block shutdown for process launch on Windows

Launching a process is racy with browser shutdown, and blocks it. So we
see shutdown hangs when process launch takes a long time. This CL makes
the process launcher use a CONTINUE_ON_SHUTDOWN instead of
BLOCK_SHUTDOWN task on Windows where it's presumably safe to do so given
no non-leaky globals are used, and based on local testing. This is not
safe on Linux since the process launcher there uses SandboxHostLinux
which is a non-leaky base::Singleton.

R=brucedawson,gab
BUG=830954

Change-Id: Ia8c5295924288819dcfdcc087d282a05e6ee7e55
Reviewed-on: https://chromium-review.googlesource.com/1020720Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553294}

content/browser/child_process_launcher_helper.cc

@@ -208,13 +208,25 @@ base::SingleThreadTaskRunner* GetProcessLauncherTaskRunner() {
       launcher_task_runner(
           android::LauncherThread::GetMessageLoop()->task_runner());
   return (*launcher_task_runner).get();
-#else   // defined(OS_ANDROID)
-  // TODO(http://crbug.com/820200): Investigate whether we could use
-  // SequencedTaskRunner on platforms other than Windows.
+#else  // defined(OS_ANDROID)
+// TODO(http://crbug.com/820200): Investigate whether we could use
+// SequencedTaskRunner on platforms other than Windows.
+
+// Do not block shutdown as process creation can take a while and be reported as
+// a hang. This doesn't work on Linux because of SandboxHostLinux non-leaky
+// singleton, so only enable this on Windows where the hang was reported, and
+// it is safe to do so.
+#if defined(OS_WIN)
+  constexpr base::TaskShutdownBehavior shutdown_behavior =
+      base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN;
+#else
+  constexpr base::TaskShutdownBehavior shutdown_behavior =
+      base::TaskShutdownBehavior::BLOCK_SHUTDOWN;
+#endif  // defined(OS_WIN)
   static base::LazySingleThreadTaskRunner launcher_task_runner =
       LAZY_SINGLE_THREAD_TASK_RUNNER_INITIALIZER(
           base::TaskTraits({base::MayBlock(), base::TaskPriority::USER_BLOCKING,
-                            base::TaskShutdownBehavior::BLOCK_SHUTDOWN}),
+                            shutdown_behavior}),
           base::SingleThreadTaskRunnerThreadMode::DEDICATED);
   return launcher_task_runner.Get().get();
 #endif  // defined(OS_ANDROID)