Have TreeScopeStyleSheetCollection::document() return a reference.

Have TresScopeStyleSheetCollection::document() return a reference instead
of pointer as TreeScope::document() cannot return NULL.

Review URL: https://codereview.chromium.org/148703004

git-svn-id: svn://svn.chromium.org/blink/trunk@166297 bbb929c8-8fbe-4397-9dbb-9b2b20218538