Don't initialize SpeedTracerLogger thread or process time keepers if not enabled.

Review at http://gwt-code-reviews.appspot.com/1456801

Review by: jhumphries@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10296 8db76d5a-ed1c-0410-87a9-c151d255dfc7