CPP: Add base/thread_checker.h.

This lets make sure that the library is only called in Chromium from the UI
thread.

BUG=http://crbug.com/236272
R=jia.shao.peng@gmail.com

Review URL: https://codereview.appspot.com/9048043

git-svn-id: http://libphonenumber.googlecode.com/svn/trunk/cpp/src/phonenumbers@570 ee073f10-1060-11df-b6a4-87a95322a99c
3 files changed