Remove dangling process launcher thread reference.

This thread was recently removed, and is no longer used.

BUG=815225

Change-Id: I11e14933e9f30791be835034f59296bdbbedb708
Reviewed-on: https://chromium-review.googlesource.com/963597
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543708}

content/public/test/test_browser_thread_bundle.h

@@ -130,7 +130,6 @@ class TestBrowserThreadBundle {
 
   std::unique_ptr<base::test::ScopedTaskEnvironment> scoped_task_environment_;
   std::unique_ptr<TestBrowserThread> ui_thread_;
-  std::unique_ptr<TestBrowserThread> process_launcher_thread_;
   std::unique_ptr<TestBrowserThread> io_thread_;
 
   int options_;