Oilpan: Replace TreeScope& in TreeScopeStyleSheetCollection with RawPtrWillBeMember<TreeScope>

BUG=340522

Review URL: https://codereview.chromium.org/438743002

git-svn-id: svn://svn.chromium.org/blink/trunk@179359 bbb929c8-8fbe-4397-9dbb-9b2b20218538