StyleSheetContents doesn't need a set of clients.

The clients were only used to call clearResolver(), but all clients in Sky
are from the same document, so we can just store the document instead. We
can also use the lifetime of the StyleSheetContents object to manage the
cache in the StyleEngine instead of the list of clients.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/791633010