Create a reader-writer lock implementaiton in base.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index fcd6803a6e0af6c061e3cbfed58c01b35ac8b9c0..710bcbfedd9c19f78a39bc17e1b9110e45dff938 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -730,6 +730,10 @@ component("base") {
     "synchronization/lock_impl.h",
     "synchronization/lock_impl_posix.cc",
     "synchronization/lock_impl_win.cc",
+    "synchronization/rw_lock.h",
+    "synchronization/rw_lock_nacl.cc",
+    "synchronization/rw_lock_posix.cc",
+    "synchronization/rw_lock_win.cc",
     "synchronization/spin_wait.h",
     "synchronization/waitable_event.h",
     "synchronization/waitable_event_posix.cc",
@@ -1122,6 +1126,7 @@ component("base") {
       "process/process_posix.cc",
       "scoped_native_library.cc",
       "sync_socket_posix.cc",
+      "synchronization/rw_lock_posix.cc",
       "sys_info.cc",
       "sys_info_posix.cc",
       "trace_event/trace_event_system_stats_monitor.cc",
@@ -1156,6 +1161,7 @@ component("base") {
       "os_compat_nacl.cc",
       "os_compat_nacl.h",
       "rand_util_nacl.cc",
+      "synchronization/rw_lock_nacl.cc",
     ]
   }
 
@@ -1852,6 +1858,7 @@ test("base_unittests") {
     "synchronization/cancellation_flag_unittest.cc",
     "synchronization/condition_variable_unittest.cc",
     "synchronization/lock_unittest.cc",
+    "synchronization/rw_lock_unittest.cc",
     "synchronization/waitable_event_unittest.cc",
     "synchronization/waitable_event_watcher_unittest.cc",
     "sys_info_unittest.cc",