Do not allow sync primitives on the process launcher thread.

It was merely added to match previous allowance before
migration to TaskScheduler, but this allowance doesn't
appear to be needed (bots are happy without).

Bug: 815225
Change-Id: Ie8ab830fe3807ed29fa91ea06c30b326082e93c6
Reviewed-on: https://chromium-review.googlesource.com/961455Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543355}

content/browser/child_process_launcher_helper.cc

@@ -207,10 +207,8 @@ base::SingleThreadTaskRunner* GetProcessLauncherTaskRunner() {
       launcher_task_runner(
           android::LauncherThread::GetMessageLoop()->task_runner());
 #else   // defined(OS_ANDROID)
-  // TODO(gab): WithBaseSyncPrimitives() is likely not required here.
   constexpr base::TaskTraits task_traits = {
-      base::MayBlock(), base::WithBaseSyncPrimitives(),
-      base::TaskPriority::USER_BLOCKING,
+      base::MayBlock(), base::TaskPriority::USER_BLOCKING,
       base::TaskShutdownBehavior::BLOCK_SHUTDOWN};
   // TODO(wez): Investigates whether we could use SequencedTaskRunner on
   // platforms other than Windows. http://crbug.com/820200.