Add DIR_NATIVE_MESSAGING and DIR_USER_NATIVE_MESSAGING paths.

Previously the paths for DIR_NATIVE_MESSAGING were hardcoded in
native_process_launcher_posix.cc.
DIR_USER_NATIVE_MESSAGING will be used for user-level Native Messaging
support.

BUG=237873

Review URL: https://codereview.chromium.org/140423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246561 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246561