Refactoring: Extract StyleSheetCandidate from DocumentStyleSheetCollection

This change introduces StyleSheetCandidate that encapsulates
difference between candidate node subclasses.

TEST=none
BUG=334338
R=tasak@google.com, dglazkov, tasak

Review URL: https://codereview.chromium.org/139273002

git-svn-id: svn://svn.chromium.org/blink/trunk@165196 bbb929c8-8fbe-4397-9dbb-9b2b20218538