drive: Remove ChangeListProcessor::ConvertToMap

What ConvertToMap does is just storing entries to a std::map.
There is no reason to use the same implementation for both tests and production code.

BUG=275270
TEST=unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/25852002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226962 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226962