Add a sequenced worker pool.

This allows tasks to be put in the worker pool with optional sequencing semantics for consumers that must run a bunch of stuff in order on a background thread, but don't particularly care about which thread.
Review URL: http://codereview.chromium.org/8416019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@116078 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/base.gyp b/base/base.gyp
index a57fc7bc..a5c045ce 100644
--- a/base/base.gyp
+++ b/base/base.gyp
@@ -224,6 +224,7 @@
         'test/trace_event_analyzer_unittest.cc',
         'threading/non_thread_safe_unittest.cc',
         'threading/platform_thread_unittest.cc',
+        'threading/sequenced_worker_pool_unittest.cc',
         'threading/simple_thread_unittest.cc',
         'threading/thread_checker_unittest.cc',
         'threading/thread_collision_warner_unittest.cc',