GitLab

This CL adds GetChangelist operation and its corresponding interface
for DocumentsService.  No code uses this interface yet.

BUG=chromium:127728
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10836125

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150322 0039d316-1c4b-4281-b951-d872f2087c98