Make DefaultComponentInstallerTest's SequencedWorkerPool multi-threaded.

BUG=646443
NO_DEPENDENCY_CHECKS=true

Review-Url: https://codereview.chromium.org/2338333002
Cr-Commit-Position: refs/heads/master@{#419070}

components/component_updater/default_component_installer_unittest.cc

@@ -132,7 +132,7 @@ class DefaultComponentInstallerTest : public testing::Test {
   void RunThreads();
 
  private:
-  static const int kNumWorkerThreads_ = 1;
+  static const int kNumWorkerThreads_ = 2;
 
   base::MessageLoopForUI message_loop_;
   base::RunLoop runloop_;