Renames error codes to be more like enums.

BUG=328120

Review URL: https://codereview.chromium.org/114433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241652 0039d316-1c4b-4281-b951-d872f2087c98
[sync] Rename migration_errors_test.cc to migration_test.cc

This CL takes care of a TODO and renames migration_errors_test.cc to
migration_test.cc.

BUG=none
TEST=sync_integration_tests:*Migration*
R=rlarocque@chromium.org

Review URL: https://codereview.chromium.org/104503012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241653 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241653