Add PBChangeSourceWithComparator.
Add PBChangeSourceWithComparator.

This is needed to render the console view when there are changes
from a PBChangeSource.

BUG=389279
TBR=iannucci@chromium.org,mmoss@chromium.org,petermayo@chromium.org

Review URL: https://codereview.chromium.org/493233002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@291216 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build