Begin the first small step towards using FilePath everywhere:
- Add some transition APIs.
- Start migrating some code to transition APIs.

Review URL: http://codereview.chromium.org/8825


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@4254 0039d316-1c4b-4281-b951-d872f2087c98

• base/base_paths.cc[diff]
• base/base_paths_linux.cc[diff]
• base/file_path.cc[diff]
• base/file_path.h[diff]
• base/file_util.cc[diff]
• base/file_util.h[diff]
• base/file_util_linux.cc[diff]
• base/file_util_mac.mm[diff]
• base/file_util_posix.cc[diff]
• base/file_util_unittest.cc[diff]
• base/file_util_win.cc[diff]
• base/icu_util.cc[diff]
• base/path_service.cc[diff]
• base/path_service.h[diff]
• base/path_service_unittest.cc[diff]
• base/trace_event.cc[diff]
• chrome/browser/download/download_uitest.cc[diff]
• chrome/browser/download/save_page_uitest.cc[diff]