Rename StyleSheetCollections to StyleEngine.

This will be the new state/controller for everything related to style resolution.

BUG=291287
R=abarth, eae, eseidel

Review URL: https://chromiumcodereview.appspot.com/23516012

git-svn-id: svn://svn.chromium.org/blink/trunk@157837 bbb929c8-8fbe-4397-9dbb-9b2b20218538