Also I believe we don't need this check anymore.
context.getLibrariesContaining(source).isEmpty &&
context.getLibrariesDependingOn(source).isEmpty)
1. It was not used in applyChanges(), which could probably cause losing
some dependencies.
2. Since https://codereview.chromium.org/2172143003 we record that
there are dependencies on the removed, so not existing, source. So,
when the source is added again, _sourceAvailable() will invalidate
its CONTENT and the whole transitive closure of its users.
R=brianwilkerson@google.com
BUG=
Review URL: https://codereview.chromium.org/2209493003 .
Konstantin Shcheglov
6cceb72ea7