Renamed DocumentStyleSheetCollection to StyleSheetCollections.

BUG=244314

TEST=No tests, because just renaming.

Review URL: https://chromiumcodereview.appspot.com/23068031

git-svn-id: svn://svn.chromium.org/blink/trunk@156829 bbb929c8-8fbe-4397-9dbb-9b2b20218538