Add NeedsManualRebaseline.

This is the same as NeedsRebaseline except rebaseline-o-matic doesn't
automatically rebaseline it. This is useful for things like two-sided
patches, where you still need to add lines to TestExpectations but
don't want to add [ Pass Failure ImageOnlyFailure Missing ] lines.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/18889007

git-svn-id: svn://svn.chromium.org/blink/trunk@154146 bbb929c8-8fbe-4397-9dbb-9b2b20218538