Make DefaultComponentInstallerTest's SequencedWorkerPool multi-threaded.

components/component_updater/default_component_installer_unittest.cc

Index: components/component_updater/default_component_installer_unittest.cc
diff --git a/components/component_updater/default_component_installer_unittest.cc b/components/component_updater/default_component_installer_unittest.cc
index 3fe484da80fb450dfe0c75efe402a2da95fd94c8..d235714610cf1be7539cb308f162eddfa8cace8a 100644
--- a/components/component_updater/default_component_installer_unittest.cc
+++ b/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_;