Avoid modification during iteration in StyleSheetContents::startLoadingDynamicSheet.

Also remove oilpan related comment in checkLoaded that is no longer
valid with the split in completed and loading clients. The copy to
a vector guards against modification during iteration and will be
needed with oilpan as well.

R=esprehn@chromium.org, tasak@chromium.org
BUG=352941

Review URL: https://codereview.chromium.org/201703002

git-svn-id: svn://svn.chromium.org/blink/trunk@169346 bbb929c8-8fbe-4397-9dbb-9b2b20218538