This change will allow projects other than chromium to specify 'BUG_PREFIX: project_name:' in their codereview.settings file.
Currently, the change is created with 'BUG=', entering a project's bug number there will point it to chromium's issue tracker. Adding a project specific prefix will make the links point to the project's issue tracker without the developer having to know about the magic prefix.

BUG=skia:1983

Review URL: https://codereview.chromium.org/132233006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@244801 0039d316-1c4b-4281-b951-d872f2087c98