Mojo Log service and a thread-safe client library.
The service impl itself is pretty thin and does what we currently already do:  print messages to stderr.
The log client provides a MojoLogger (you can hook this up to mojo::Environment and have MOJO_LOG() go directly to the Log service), and is thread-safe.

Other small changes to acommodate this CL:
 - mojo::Environment now has SetDefaultLogger() for replacing the logger.
 - Move mojo::internal::MessageBuilder out of internal namespace.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1447273002 .
diff --git a/services/BUILD.gn b/services/BUILD.gn
index 419c423..919f3f0 100644
--- a/services/BUILD.gn
+++ b/services/BUILD.gn
@@ -17,6 +17,7 @@
     "//services/gles2:lib",
     "//services/http_server",
     "//services/icu_data",
+    "//services/log",
     "//services/media",
     "//services/native_support",
     "//services/prediction",
@@ -111,6 +112,7 @@
   if (is_android || is_linux) {
     deps += [
       "//services/files:apptests",
+      "//services/log:apptests",
       "//services/url_response_disk_cache:tests",
       "//services/view_manager:mojo_view_manager_client_apptests",
       "//services/view_manager:view_manager_service_apptests",