Move SimpleStringDictionary from common/mac/ to just common/.

This also cleans up some things like the file name, trailing whitespace,
and making the test use gtest instead of sentest, since there's nothing
Mac specific about this.

BUG=https://code.google.com/p/chromium/issues/detail?id=77656

Review URL: https://breakpad.appspot.com/561003

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1154 4c0a9323-5329-0410-9bdc-e9ce6186880e