Have StyleSheetCollections constructor take a Document reference in argument

Have StyleSheetCollections and StyleSheetCollection constructors take a Document
reference in argument instead of a pointer since it can never be NULL.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23851003

git-svn-id: svn://svn.chromium.org/blink/trunk@157050 bbb929c8-8fbe-4397-9dbb-9b2b20218538