Consistently use on-heap collections for StyleRuleBase descendants.

StyleRuleBase and StyleSheetContents are on the heap now so off heap collections of raw pointers to them are disallowed.

BUG=341815

Review URL: https://codereview.chromium.org/180003002

git-svn-id: svn://svn.chromium.org/blink/trunk@167916 bbb929c8-8fbe-4397-9dbb-9b2b20218538