Logger: remove dependency between Logger and LogMessageBuilder.

LogMessageBuilder is a helper class for Log.
So I made it a nested class and removed the dependency from Logger.

BUG=none
TEST=no changes in the logic
R=yangguo@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/19768003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15760 ce2b1a6d-e550-0410-aec6-3dcde31c8c00