Refactoring: Extract StyleSheetCollectionBase from StyleSheetCollection.

This chagne extract StyleSheetCollection bas which carries
two list of stylesheet but nothing else. This class makes sense
since these two lists are the unit of the update operation.
Also, coming change will use this as a unit of cross-document
active sheet collection.

BUG=319151
TEST=none
R=tasak@google.com, esprehn, tasak

Review URL: https://codereview.chromium.org/80243002

git-svn-id: svn://svn.chromium.org/blink/trunk@162443 bbb929c8-8fbe-4397-9dbb-9b2b20218538